Add DescribeWorker API (#624)

<!-- Describe what has changed in this PR -->
**What changed?**
Add DescribeWorker API.
Extracted from [this PR](temporalio/api#622).

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DescribeWorker - will be needed to get all the heartbeat information
from the worker.
Partially this can be covered by "ListWorkers" with query
"WorkerInstanceKey = 'xxx'", but this is a dedicated API that doesn't
require query processing.

Extracting into separate PR so that one is concentrated on worker
payload only.

<!-- Are there any breaking changes on binary or code level? -->
**Breaking changes**
Introduce new API

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right after this PR was merged. -->
[Server PR](temporalio/temporal#8094)

Author: ychebotarev