KAFKA-20790: Promote streams assignor API to public module

[gabriellefu]

Changes

New public API package

org.apache.kafka.coordinator.group.api.assignor.streams (in group-coordinator-api), all annotated with @InterfaceAudience.Public and @InterfaceStability.Evolving.

• Moved TaskAssignor, GroupAssignment, GroupSpec, MemberAssignment, MemberSubscription, TaskId, TopologyDescriber, and TaskAssignorException from the internal streams.assignor package into the new public package.
• Added package-info.java.
• Reshaped the input/output contracts to match the KIP:
  • GroupSpec now exposes memberIds(), memberSubscription(id), memberAssignmentState(id), and configs() instead of members() and assignmentConfigs().
  • MemberSubscription is now a thin interface containing instanceId, rackId, processId, and clientTags.
  • Introduced MemberAssignmentState with activeTasks, standbyTasks, warmupTasks, taskOffsets, and taskEndOffsets to represent a member’s current assignment state, split out from the previous larger member spec.
  • MemberAssignment is now an interface exposing only activeTasks() and standbyTasks(). Warm-up tasks are no longer produced by the assignor and are now determined during reconciliation.

Internal implementations

These remain in group-coordinator, under the streams.assignor package.

• Added MemberSubscriptionAndAssignmentImpl, which implements both MemberSubscription and MemberAssignmentState.
• Added MemberAssignmentImpl, which implements MemberAssignment.
• Removed the old AssignmentMemberSpec record.
• Reworked GroupSpecImpl to back the new GroupSpec interface.
• Updated the built-in assignors (StickyTaskAssignor, MockAssignor) and TargetAssignmentBuilder to use the new interfaces.
• TargetAssignmentBuilder now always emits empty warm-up tasks in the resulting target assignment.
• Updated import paths in GroupMetadataManager, TopologyMetadata, MemberTaskOffsets, and ProcessState.

Tests and benchmarks

• Updated the streams assignor tests, TargetAssignmentBuilderTest, GroupMetadataManagerTest, and the JMH benchmarks (StreamsAssignorBenchmarkUtils, StreamsStickyAssignorBenchmark) to use the new API.
• Rewrote GroupSpecImplTest to exercise the new accessors.
• Restricted the output-asserting parameterized tests (testAssignmentHasNotChanged, testAssignmentSwapped, testPartialAssignmentUpdate) to ACTIVE and STANDBY, since warm-up tasks can no longer appear in assignor output.