[Draft] Add support for standalone callbacks

[chrsmith]

ℹ️ This PR is a rebase and cleanup from an earlier prototype, #9805.

⚠️ This is dependent on changes in the api and api-go repos. And will need some care before merging.

What changed?

Adds support for "standalone" callbacks. Today, the CHASM Callback component is used to deliver an arbitrary payload to a URL. (e.g. when a Workflow has completed.) As part of supplying the Nexus Connector Foundations, this feature adds CRUD operations on callbacks directly. So callers can invoke StartCallbackExecution(...) and get the durability guarantees to ensure that the callback actually gets invoked.

Why?

The primary (only?) use-case for this is for completing Nexus operations. With this capability, a Handler can implement a Nexus operation outside of Temporal. And when that operation completes, simply call StartCallbackExecution(...) with the right callback URL and Token. Then the CHASM Callback machinery will attempt to deliver the result for the Nexus operation. (Rather than, say, the Nexus operation to be implemented as a Workflow that is separately polling the async or out-of-band process.)

How did you test it?

• built
• run locally and tested manually
• covered by existing tests
• added new unit test(s)
• added new functional test(s)

Potential risks

TBD. Will need to ask around.