[Improvement] Implement commitTransaction REST endpoint for Iceberg REST catalog

[laserninja]

What would you like to be improved?

Implement the missing POST /v1/{prefix}/transactions/commit REST endpoint in Gravitino's Iceberg REST catalog server, as defined by the Iceberg REST Catalog API spec (CommitTransactionRequest).

The Iceberg REST spec defines POST /v1/{prefix}/transactions/commit for atomic multi-table commits. Gravitino's Iceberg REST server does not implement this endpoint. Clients performing atomic cross-table operations (e.g., Spark multi-table writes) will fail when targeting Gravitino as an Iceberg REST catalog.

How should we improve?

• Add IcebergTransactionOperations JAX-RS class with POST /v1/{prefix}/transactions/commit
• Add IcebergTransactionOperationDispatcher interface + Executor/EventDispatcher/HookDispatcher chain
• Wire the endpoint into the existing server bootstrap
• Register the endpoint in ConfigResponse so clients discover it
• Add unit and integration tests