Use a datasource schema for Spanner Instance DS

[rileykarson]

This didn't seem to cause a failure before but does now w/ the changes to node_count and processing_units. The datasourceSchema call was missing in the initial impl in #4111

$ make testacc TEST=./google-beta TESTARGS='-run=TestAccDataSourceSpannerInstance_basic'
==> Checking source code against gofmt...
==> Checking that code complies with gofmt requirements...
TF_ACC=1 TF_SCHEMA_PANIC_ON_ERROR=1 go test ./google-beta -v -run=TestAccDataSourceSpannerInstance_basic -timeout 240m -ldflags="-X=github.com/hashicorp/terraform-provider-google-beta/version.ProviderVersion=acc"
=== RUN   TestAccDataSourceSpannerInstance_basic
=== PAUSE TestAccDataSourceSpannerInstance_basic
=== CONT  TestAccDataSourceSpannerInstance_basic
--- PASS: TestAccDataSourceSpannerInstance_basic (24.40s)
PASS
ok  	github.com/hashicorp/terraform-provider-google-beta/google-beta	25.776s

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Release Note Template for Downstream PRs (will be copied)

spanner: fixed the schema for `data.google_spanner_instance` so that non-configurable fields are considered outputs

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 2 files changed, 14 insertions(+), 5 deletions(-))
Terraform Beta: Diff ( 2 files changed, 14 insertions(+), 5 deletions(-))