[rest] Add database branch and tag APIs

[JingsongLi]

Purpose

Add database-level writable branches and immutable tags to the REST client and protocol, exposed through RESTCatalog.treeManagement(). Callers can page through references, inspect a reference, create a branch or tag from an existing reference, fast-forward a branch to an immutable tag, and delete references.

• Introduce TreeManagement and RESTTreeManagement, reusing the catalog's configured HTTP client, authentication, and prefix.
• Add the /trees endpoints, reference models, DatabaseReferenceResponse, pagination, and HTTP client support to the REST OpenAPI specification and client.
• Use dedicated JSON request objects for create, fast-forward, and delete. GET filtering and pagination use query parameters.
• Expose fast-forward as POST /v1/{prefix}/databases/{database}/trees/{name}/forward, following the table-level forward operation's path and HTTP method.

This PR provides the reference-management client and protocol surface. Reference persistence requires a catalog server implementation.

Tests

• mvn -pl paimon-core -am -DwildcardSuites=none -DfailIfNoTests=false -Dtest=RESTCatalogTreeManagementTest,RESTApiDatabaseReferenceTest test — 7 tests covering catalog configuration, branch/tag operations, the forward route and JSON body, pagination, and error propagation; Checkstyle, Spotless, and Enforcer enabled.
• OpenAPI YAML parse, reference, and path-parameter validation.
• git diff --check.