Complete health checks watch service on server shutting down

[JamesNK]

Fixes the same issue as #2573: health checks Watch call completes when the server begins shutting down. This avoids the problem of a shutting down server waiting for the health check call to be complete (default is 30 seconds) before forcefully killing the request.

Differences:

• Avoids duplicating the health checks service implementaiton. Cancellation is now passed in via ServerCallContext
• Adds option to enable and disable this behavior. Completing on shutdown is enabled by default with the option to suppress it and go back to old behavior.
• Adds tests

Copilot reviewed 5 out of 6 changed files in this pull request and generated no suggestions.

Files not reviewed (1)

• src/Grpc.AspNetCore.HealthChecks/GrpcHealthChecksOptions.cs: Evaluated as low risk

Comments skipped due to low confidence (2)

src/Grpc.HealthCheck/HealthServiceImpl.cs:144

• The word 'implementaiton' in the pull request description is misspelled. It should be 'implementation'.

// The call has already been canceled. Writing to the response will fail so immediately exit.

src/Grpc.AspNetCore.HealthChecks/GrpcHealthChecksPublisher.cs:18

• The word 'implementaiton' is misspelled. It should be 'implementation'.

Avoids duplicating the health checks service implementaiton. Cancellation is now passed in via ServerCallContext