[server] Support group-aware tiering table assignment - #4745

Purpose

Linked issue: close #3789

Allow operators to assign tables to separate tiering services so workloads with different resource or freshness requirements can be scaled independently. Services in the same group continue to share work.

Brief change log

• Add the mutable table option table.datalake.tiering-group and service option --fluss.tiering.group, both defaulting to default, with shared group-name validation.
• Assign pending tables only to services in the matching group. Apply group changes asynchronously through the existing metadata update path, allowing an already assigned round to finish with its current epoch.
• Document group configuration, reassignment behavior, and rolling-upgrade compatibility; cover group-aware scheduling, heartbeat compatibility, and real Flink-to-Paimon tiering in tests.
• Regenerate the checked-in Rust protobuf bindings for the heartbeat field with the CI-pinned generator.

Tests

Passed 90 unique targeted test cases after integrating apache/main at b25ba46d8:

• TableConfigTest, TableDescriptorValidationTest
• ServerConnectionTest
• LakeTableTieringManagerTest, LakeTieringHeartbeatITCase
• TieringSourceEnumeratorSchedulingTest, TieringSourceEnumeratorTest, TieringCommitOperatorTest
• FlussLakeTieringTest
• PaimonTieringITCase#testTieringWithGroups and the existing testTiering end-to-end case

The affected modules and their full dependency reactor passed Maven verify on JDK 17, including Spotless, Checkstyle, and validation checks. Both Paimon end-to-end cases passed twice after integrating the upstream tiering refactor. The full repository test suite has not been run.

Rust protobuf regeneration is deterministic with protoc 27.1, and cargo check --package fluss-rs --lib --locked --offline passed.

API and Format

Add the optional tiering_group field to the internal lake-tiering heartbeat request and advertise heartbeat protocol version 1 while retaining version 0 support. An absent group means default; non-default group requests are rejected before being sent to an older Coordinator. Storage formats are unchanged.

Documentation

Update website/docs/streaming-lakehouse/tiering-service.md with group configuration, dynamic reassignment, default behavior, legacy-table enablement, and upgrade/downgrade guidance.