Merge initial protos for priority (#561)

**What changed?**
Add initial protos for priority, enough to attach simple priority
metadata to workflows and activities.

See #513 for the full future plans, only the first field (priority_key)
is included so far.

**Why?**
Control task ordering.

**Server PR**
Implementation on `priority` branch:
temporalio/temporal@main...priority

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Co-authored-by: Spencer Judge <[email protected]>
Co-authored-by: Yuri <[email protected]>
Co-authored-by: Rodrigo Zhou <[email protected]>
Co-authored-by: Carly de Frondeville <[email protected]>
Co-authored-by: Spencer Judge <[email protected]>

Author: 5 people