Job cancellation on remote builds

Background

Buildstream should try and cancel remote jobs on termination.

Task description

This should just require sending the CancelOperation rpc, which may not always be implemented. Buildgrid may need changes to make sure this works.

Acceptance Criteria

Sends a CancelOperation rpc when a user selects to terminate, and handles a grpc unimplemented error when it's not been implemented on the server side.

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