datalake_agent: Polaris (REST) catalog #2015

Part of #2008. Letters (A, B0–B7, C, D, E) are the PRs listed there; this is B5.

Scope

The first external catalog provider.

• RESTCatalog: OAuth2 client credentials or bearer token, warehouse, vended credentials so a volume can be omitted. polaris accepted as an alias of rest, as in the fdw whitelist.
• Commit through the catalog's CAS; B3's operation_id store covers the in-doubt window between "committed" and "the client heard".
• Drop with and without purge; namespaces through CatalogManagementService.
• End-to-end against Apache Polaris (or the Iceberg REST fixture image) plus MinIO in docker.

Out of scope

Hive, Hadoop. Anything the REST spec leaves optional and Polaris does not serve.

Depends on

B3.

Acceptance

• Conformance suite green against Polaris; a table Cloudberry creates is visible to Spark in the same catalog, and the reverse.
• DROP TABLE with and without purge_on_drop behaves as the option says.
• A commit retried after a simulated timeout yields exactly one snapshot.