[VegetarianOrc]

What was changed

• NexusClient.start_operation and NexusClient.execute_operation accept a new completion_callbacks argument.
• The new function temporalio.nexus.create_completion_callback creates a CompletionCallback. It accepts a service name and an operation name, or an on-complete operation from a service definition or a handler method. It checks that the operation takes OnCompleteRequest as input and returns OnCompleteResponse.
• Type checkers check that each callback accepts the output type of the operation. A handler that receives RawValue accepts the output of any operation.
• The payload visitor visits the OnCompleteRequest and OnCompleteResponse messages of the notification service.
• The CI workflow installs NexGen 0.2.7. The tests use the v1.9.2-nexus-handler-callbacks dev server.

Why

A caller of a standalone Nexus operation can now get the outcome of the operation in a Nexus handler. When the operation finishes, the server calls an on-complete operation on a worker for the task queue of the callback. The call contains the result or the failure, and the source context that the caller supplies.

Checklist

1. How was this tested:

• New tests deliver success, failure, cancellation and termination outcomes to on-complete handlers on a worker.
• New type tests check the overloads of create_completion_callback.

2. Any docs updates?

Yes, new docs will be added in support of the Nexus handler callbacks feature.

[tconley1428]

Just have to remove the changelog modification.