Support attaching to non-idempotent invocations

[tillrohrmann]

Right now, we explicitly forbid attaching to non-idempotent invocations. It is not 100% clear whether there is a strong requirement for it because attaching to a non-idempotent invocation based on its InvocationId should be technically the same as attaching to an idempotent invocation with 0 retention. If the invocation is still pending or running, then we update the ResponseSink to include another sink. If the invocation is not present, then we return a NOT_FOUND_INVOCATION_ERROR.

Originally, we forbid attaching to non-idempotent invocations via #1583. I might have been a remnant from previous times which is no longer valid.