Add links and callbacks support for standalone activity (#759)

**What changed?**
- Added completion_callbacks and links fields to
StartActivityExecutionRequest.
- Added link field to StartActivityExecutionResponse.
  - Added callbacks field to DescribeActivityExecutionResponse.
- Added links field to ActivityExecutionInfo.
  - Added new Link.Activity variant to the Link message type.
- Added new CallbackInfo base message
  - Updated OpenAPI v2/v3 specs to reflect the above changes.

**Why?**
To support links and callbacks for standalone activity executions (SAA),
bringing feature parity with workflow-based activity scheduling.

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Co-authored-by: Quinn Klassen <klassenq@gmail.com>
Co-authored-by: Spencer Judge <spencer@temporal.io>

Author: 3 people