Update Sync Association resource to include sync metadata for all subkeys

[vinay-gopalan]

Description

Previously, the association resource did not account for the secret-key granularity, which would create a sync target for each subkey of a KV secret. Now, the association resource must keep track of the sync statuses for all subkeys of this associated secret.

Notable improvements/changes:

• Adds a new list schema field metadata which tracks the association metadata for each subkey (sync_status, updated_at)
• Removes the schema types sync_status and updated_at
• Adds granularity to all destination resources.
• Makes the Association resource importable. Now, by providing the ID with the right format (encoding all necessary info), TFVP can enable a user to import their association metadata to track the lifecycle of all subkeys of an associated secret.
• Fixes some tests

Checklist

• [x] Added CHANGELOG entry (only for user-facing changes)
• [x] Acceptance tests where run against all supported Vault Versions

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestSecretsSyncAssociation_gh'
=== RUN   TestSecretsSyncAssociation_gh
    resource_secrets_sync_association_test.go:38: Vault server version "1.16.0+ent"
--- PASS: TestSecretsSyncAssociation_gh (2.67s)
PASS

[raymonstah]

LGTM!