Add Spanner instance datasource

[upodroid]

Fixes: hashicorp/terraform-provider-google#1866

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

`google_spanner_instance`

[modular-magician]

Hello! I am a robot who works on Magic Modules PRs.

I have detected that you are a community contributor, so your PR will be assigned to someone with a commit-bit on this repo for initial review.

Thanks for your contribution! A human will be with you soon.

@slevenick, please review this PR or find an appropriate assignee.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 5 files changed, 116 insertions(+))
Terraform Beta: Diff ( 5 files changed, 116 insertions(+))

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 5 files changed, 116 insertions(+))
Terraform Beta: Diff ( 5 files changed, 116 insertions(+))

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 5 files changed, 116 insertions(+))
Terraform Beta: Diff ( 5 files changed, 116 insertions(+))

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 5 files changed, 116 insertions(+))
Terraform Beta: Diff ( 5 files changed, 116 insertions(+))

[slevenick]

/gcbrun

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=152270"

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 5 files changed, 116 insertions(+))
Terraform Beta: Diff ( 5 files changed, 116 insertions(+))

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccDataSourceGoogleComputeInstanceGroup_basic|TestAccDataSourceComputeInstanceSerialPort_basic|TestAccDataSourceGoogleComputeInstanceGroup_withNamedPort|TestAccDataSourceGoogleComputeInstanceGroup_fromIGM|TestAccDataSourceSpannerInstance_basic|TestAccComputeInstanceIamBindingGenerated|TestAccComputeInstanceIamMemberGenerated|TestAccComputeInstanceIamMemberGenerated_withCondition|TestAccComputeInstanceIamBindingGenerated_withCondition|TestAccComputeInstanceIamPolicyGenerated|TestAccComputeInstanceIamPolicyGenerated_withCondition|TestAccIapTunnelInstanceIamBindingGenerated|TestAccIapTunnelInstanceIamMemberGenerated|TestAccIapTunnelInstanceIamBindingGenerated_withCondition|TestAccIapTunnelInstanceIamPolicyGenerated|TestAccIapTunnelInstanceIamMemberGenerated_withCondition|TestAccIapTunnelInstanceIamPolicyGenerated_withCondition|TestAccProviderMeta_setModuleName|TestAccComputeAutoscaler_autoscalerSingleInstanceExample|TestAccComputeAutoscaler_autoscalerBasicExample|TestAccComputeAutoscaler_update|TestAccComputeAutoscaler_multicondition|TestAccComputeAutoscaler_scaleDownControl|TestAccComputeForwardingRule_forwardingRuleHttpLbExample|TestAccComputeGlobalForwardingRule_globalForwardingRuleInternalExample|TestAccInstanceGroupManager_targetSizeZero|TestAccInstanceGroupManager_basic|TestAccInstanceGroupManager_update|TestAccInstanceGroupManager_versions|TestAccInstanceGroupManager_autoHealingPolicies|TestAccInstanceGroupManager_stateful|TestAccComputeInstanceGroup_update|TestAccComputeInstanceGroup_rename|TestAccComputeInstanceGroup_basic|TestAccComputeInstanceGroup_network|TestAccComputeInstanceGroup_outOfOrderInstances|TestAccComputeInstanceIamPolicy|TestAccComputeMachineImage_machineImageBasicExample|TestAccComputePacketMirroring_computePacketMirroringFullExample|TestAccComputePerInstanceConfig_update|TestAccComputeRegionAutoscaler_regionAutoscalerBasicExample|TestAccComputeRegionAutoscaler_update|TestAccComputeRegionAutoscaler_scaleDownControl|TestAccComputeRegionBackendService_regionBackendServiceBalancingModeExample|TestAccComputeRegionBackendService_withBackendInternal|TestAccComputeRegionBackendService_withBackendMultiNic|TestAccComputeRegionBackendService_withBackendInternalManaged|TestAccComputeRegionBackendService_ilbUpdateFull|TestAccComputeRegionDisk_deleteDetach|TestAccRegionInstanceGroupManager_basic|TestAccRegionInstanceGroupManager_targetSizeZero|TestAccRegionInstanceGroupManager_update|TestAccRegionInstanceGroupManager_versions|TestAccRegionInstanceGroupManager_autoHealingPolicies|TestAccRegionInstanceGroupManager_distributionPolicy|TestAccRegionInstanceGroupManager_stateful|TestAccComputeTargetInstance_targetInstanceBasicExample|TestAccComputeTargetInstance_targetInstanceCustomNetworkExample|TestAccNetworkManagementConnectivityTest_networkManagementConnectivityTestInstancesExample|TestAccNotebooksInstance_notebookInstanceBasicExample|TestAccNotebooksInstance_notebookInstanceBasicContainerExample|TestAccOSConfigGuestPolicies_osConfigGuestPoliciesBasicExample|TestAccOSConfigPatchDeployment_osConfigPatchDeploymentInstanceExample You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=152300"

[slevenick]

Looks reasonable to me. Thanks for the contribution!