Update Workflow and Activity Priority (#610)

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<!-- Describe what has changed in this PR -->
**What changed?**

Added `Priority` field to the workflow and activity update API options.

Batch API support is going to be added in a follow-up PR.

<!-- Tell your future self why have you made these changes -->
**Why?**

Allow users to update priority after the workflow/activity was
submitted.

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

See field deprecation.

<!-- If this breaks the Server, please provide the Server PR to merge
right after this PR was merged. -->

**Server PR**

TBD

Author: stephanos