[EventHubs] migrate EventHubs tests from `azure-mgmt-eventhub` to arm template/`azure-mgmt-resource`

[swathipil]

Need to move EventHubs tests over to using the EnvVariableLoader + ARM template to create resources. Currently we're using the azure-mgmt-eventhub SDK, which means we have to add this to our dev_requirements.txt file. This limits us, when we run into issues with the mgmt sdk.

related to: #22866

Should hopefully not run into issues enabling live tests on the sovereign cloud after this: #21715

[swathipil]

update: Decided that we can't use the EngSys process of only deploying the ARM template once before running all tests. Instead, we'll need to:

1. Make sure Update EventHub to enable live testing in sovereign clouds for multiple services #21715 is merged first.
2. Look into using azure-mgmt-resource and deployment.deploy() decorator to deploy an arm template per test. azure-mgmt-resource is more stable than azure-mgmt-eventhub, which can then be removed from dev_requirements.txt.