Implementation for DescribeWorker API (#8094)

## What changed?
* Add implementation for DescribeWorker API (to both frontend and
matching)
* Add feature flag
* Add all quotas/etc.
* Add unit tests
* Add functional tests

## Why?

Part of worker commands work. 
Users should be able to directly get worker info for the worker they
need.
Corresponding API PR: temporalio/api#622

## How did you test it?
- [X] add unit tests
- [X] add functional tests

## Potential risks
None, behind feature flag

Author: ychebotarev