Add workload_pool to google_container_cluster, deprecate identity_namespace

[modular-magician]

I was stuck on how to migrate this safely while making the fields O+C, as it's an intended mechanic that identity_namespace can be set to "" to disable the feature. For better or for worse workload_identity_config is O+C and I don't want to change that here.

I went with making both fields optional, and conditionally writing the value into state based on what the user set. They can specify one, none, or both of the fields and all are valid states. If they've specified identity_namespace, they'll get a deprecation warning. This adds a diff that wouldn't be present with O+C, where a user has to approve a plan that transitions from one set of fields set to the other.

These fields are a little weird, also, they're strings but can only be set to a single project-dependent value. Otherwise I'd need a conflicts.

Some basic workflows:

• import will set identity_namespace if the value is present
• adding a new value and deleting the other will send a spurious request. GKE normally restricts updates to a single field, but that's a single top level field. The request is valid- it just won't do anything.
• Adding both will do the same request- again, they're forced to match and it'll cause a spurious update
• Unsetting both and specifying an empty block will cause Terraform to send the same update but empty, disabling the feature.

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)

container: deprecated `workload_identity_config.0.identity_namespace` and it will be removed in a future major release as it has been deprecated in the API. Use `workload_identity_config.0.workload_pool` instead. Switching your configuration from one value to the other will trigger a diff at plan time, and a spurious update.

container: added `workload_identity_config.0.workload_pool` to `google_container_cluster`

Derived from GoogleCloudPlatform/magic-modules#5317

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