Set supported PD fields on workstation_config as mutable.

[modular-magician]

This change updates several fields on google_workstations_workstation_config to be mutable. In particular:

• persistentDirectories.gcePd (the top-level PD block) can now be updated if the config has an existing PD config
• persistentDirectories.gcePd[0].reclaimPolicy can now be updated
• persistentDirectories.gcePd[0].sourceSnapshot can now be updated if the existing PD config had a value for this field

The other values are set as default_from_api even though the defaults are known, since some of them have exclusivity rules with neighboring keys.

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).
• 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).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (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)

workstations: Support update of PD fields `reclaim_policy` and `source_snapshot` on `google_workstations_workstation_config` (beta)

Derived from GoogleCloudPlatform/magic-modules#8000