apikeys: migrate google_apikeys_key from DCL to MMv1 and add annotations

[DrFaust92]

Summary

Reimplements google_apikeys_key as an MMv1 resource (products/apikeys/Key.yaml), replacing the handwritten DCL implementation, and adds support for the mutable annotations field (a long-standing request — hashicorp/terraform-provider-google#18309). Adding annotations to the DCL-generated resource was not feasible (DCL is frozen and its schema isn't vendored in magic-modules), so the resource is migrated to MMv1 first, following the established DCL→MMv1 migration path (e.g. eventarc, compute firewall policies, certificate template).

Details

• products/apikeys/Key.yaml — async (LRO) create with keyId create param, update_mask PATCH, all existing fields preserved (name, display_name, project, service_account_email, restrictions, key_string, uid), plus the new annotations (KeyValueAnnotations).
• key_string is not part of the Key GET response; it is fetched from the getKeyString sub-endpoint via a post_read custom-code hook, mirroring the previous DCL behavior.
• Soft delete: a deleted API key keeps being returned by GET (with a deleteTime) until it is purged. A decoder treats such keys as gone so Terraform no longer sees a deleted key as dangling. Because the generated destroy check does a plain GET, the example uses exclude_test and a hand-written, soft-delete-aware acceptance test is provided instead.
• deletion_policy parity is preserved automatically by MMv1's unified deletion policy (ABANDON/DELETE).
• Removes the handwritten DCL files (resource, DCL client/types, meta, sweeper, generated test) and the handwritten website doc (now generated).

Testing

Acceptance test run against a real project (create → import → update annotations/display_name/restrictions → import → soft-delete-aware destroy):

--- PASS: TestAccApikeysKey_basic (44.88s)

Import round-trips all fields (annotations ignored on import, as with labels/effective_annotations). Full provider builds and vets clean.

Release Note

apikeys: added `annotations` field to `google_apikeys_key`

[github-actions]

Googlers: For automatic test runs see go/terraform-auto-test-runs.

@c2thorn, a repository maintainer, has been assigned to review your changes. If you have not received review feedback within 2 business days, please leave a comment on this PR asking them to take a look.

You can help make sure that review is quick by doing a self-review and by running impacted tests locally.