[#12179] feat(core): Support policy-tag association

[roryqi]

What changes were proposed in this pull request?

• Add persistent policy-tag relation metadata for JDBC backends, including fresh and upgrade schemas.
• Add core service, relation cache/backend routing, manager/dispatcher APIs, and cleanup on policy/tag/metalake deletion.
• Add REST APIs to list tags for a policy and list/associate policies for a tag, with targeted tests.

Why are the changes needed?

PR #12177 defines the Policy -> Tag -> Metadata Object model. This implements the direct Policy <-> Tag association layer needed by that model.

Fix: #12179

Does this PR introduce any user-facing change?

Yes. It adds REST API support for:

• GET /metalakes/{metalake}/policies/{policy}/tags
• GET /metalakes/{metalake}/tags/{tag}/policies
• POST /metalakes/{metalake}/tags/{tag}/policies

How was this patch tested?

• ./gradlew :core:spotlessApply :server:spotlessApply
• ./gradlew :core:test --tests org.apache.gravitino.storage.relational.service.TestPolicyMetaService -PskipITs
• ./gradlew :server:test --tests org.apache.gravitino.server.web.rest.TestTagOperations --tests org.apache.gravitino.server.web.rest.TestPolicyOperations -PskipITs
• git diff --check apache/main...HEAD