Worker Heartbeat: Use Worker Grouping instead of Runtime for Key in nexus TQ (#637)

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<!-- Describe what has changed in this PR -->
**What changed?**
use `worker_grouping_key` instead of `runtime_key` for worker command
nexus task queue

<!-- Tell your future self why have you made these changes -->
**Why?**
There can be multiple clients (i.e. with different auths) within the
same runtime/namespace, so we want a more specific category for these
nexus worker command requests

<!-- Are there any breaking changes on binary or code level? -->
**Breaking changes**
No, WIP feature

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right after this PR was merged. -->
**Server PR**

Author: yuandrew