Add worker visibility API - RecordWorkerHeartbeat and ListWorkers (#601)

<!-- Describe what has changed in this PR -->
**What changed?**
What changed?
* 2 new APIs added:
   * WorkerHeartbeat
   * ListWorkerStatus
* WorkerInfo information added to:
   * ShutDownWorker request
   * Poll(Activity|Nexus|Workflow)TaskQueue requests

This is "for merge".
Here is a previous PR, where we discuss this at length:
temporalio/api#599

<!-- Tell your future self why have you made these changes -->
**Why?**
Part of the worker visibility work. Workers should be able to report
their status.
Users should be able to get information about workers.

<!-- Are there any breaking changes on binary or code level? -->
**Breaking changes**
Yes - new API is introduce.

<!-- If this breaks the Server, please provide the Server PR to merge
right after this PR was merged. -->
**Server PR**
[Server PR](temporalio/temporal#7870)

Author: ychebotarev