Add new transfer task type for cancelling activities using Nexus

[rkannan82]

Add new transfer task type for activity cancellation control:

• ActivityCancelControlTask struct with WorkerInstanceKey field
• Serialization/deserialization for persistence
• No-op executor (dispatch logic in separate PR)

What changed?

Added a new transfer task type called CancelActivityNexusTask transfer task type. This will be used to dispatch activity cancellation requests to workers via their Nexus control queue.

Main changes:
Task definition
Proto

• task.proto: New task type: TASK_TYPE_TRANSFER_CANCEL_ACTIVITY_NEXUS
• executions.proto: Task definition

Struct

• cancel_activity_nexus_task.go
• task_serializers.go: Serialization/deserialization support

Task creation

• Controlled by dynamic config: EnableActivityCancellationNexusTask
• workflow_task_completed_handler.go: Groups activities by the worker and creates 1 task per group.

Task dispatch using Nexus

• Controlled by dynamic config: EnableActivityCancellationNexusTask
• Will be implemented in separate PR.

Why?

To support activity cancellation without activity heartbeat.

Overall flow:

• [Store worker_instance_key for an activity in mutable state #9231] Store worker instance key in ActivityInfo as part of RecordActivityTaskStarted call.
• [This PR] When user cancels a workflow, create 1 or more transfer tasks. Group all activities belonging to a worker into a transfer task (for efficiency).
• [Dispatch CancelActivityNexusTask to worker #9233] Lookup the Nexus task queue for each worker, and send a Nexus operation for each transfer task.
• [SDK] Worker will receive this cancel task and cancel the running activities.

How did you test it?

• built
• run locally and tested manually
• covered by existing tests
• added new unit test(s)
• added new functional test(s)

Potential risks

No. The new task creation is disabled by default.