[flink] Add describe buckets procedure

[fhan688]

Purpose

Linked issue: close #3436

PR #4029 introduced the Admin API and RPC support for retrieving bucket metadata. This pull request exposes that functionality to Flink SQL users through the sys.describe_buckets procedure.

The procedure supports describing all buckets of a table and filtering partitioned tables with a complete or partial partition spec.

Brief change log

• Add and register the sys.describe_buckets Flink system procedure.
• Provide two procedure overloads:
  • CALL sys.describe_buckets('database.table')
  • CALL sys.describe_buckets('database.table', 'key=value[/key=value...]')
• Return the complete bucket metadata:
  • table path and table ID
  • partition ID and partition name
  • bucket ID
  • leader ID and leader epoch
  • bucket epoch
  • replicas and ISR
• Preserve the existing metadata semantics:
  • partition fields are NULL for non-partitioned tables
  • leader fields are NULL when no leader is available
  • a missing bucket epoch is returned as NULL, while -1 represents missing leader/ISR state
• Validate table paths and partition specs, including malformed input and duplicate partition keys.
• Add unit and integration coverage for non-partitioned tables, partitioned tables, partial and complete partition specs, nullable fields, invalid arguments, and propagated Admin API errors.
• Document the procedure syntax, parameters, result columns, permissions, and usage examples.

Tests

• DescribeBucketsProcedureTest: 5 tests passed.
• Flink118ProcedureITCase#testDescribeBuckets passed.
• Flink22ProcedureITCase#testDescribeBuckets passed.
• Spotless passed.
• Checkstyle passed.
• RAT passed.
• git diff --check passed.

API and Format

This pull request adds a new Flink SQL system procedure and its result schema.

It does not change the Java Admin API, RPC protocol, server implementation, or storage format. The procedure delegates to the Admin#describeBuckets APIs introduced by PR #4029.

Documentation

Added documentation for sys.describe_buckets to website/docs/engine-flink/procedures.md, including syntax, parameters, output columns, metadata semantics, permission requirements, and examples.

[fhan688]

Hi @swuferhong , this PR is the Flink procedure follow-up to #4029. It exposes the describe-buckets API through sys.describe_buckets and completes #3436. All CI checks are passing now.

Could you please take a look when you have time? Thanks!