Store worker_instance_key for an activity in mutable state

[rkannan82]

What changed?

As part of RecordActivityTaskStarted flow, store worker_instance_key for an activity in the mutable state (ActivityInfo).

Main changes:

• executions.proto: Added the new worker_instance_key field.
• mutable_state_impl.go: Update mutable state.

Why?

To support activity cancellation without activity heartbeat.

Overall flow:

• [This PR] Store worker instance key in ActivityInfo as part of RecordActivityTaskStarted call.
• [Add new transfer task type for cancelling activities using Nexus #9232] 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 via Nexus control queue #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)