add cancel-after: cancel without timing out

[zellyn]

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[jhump]

Hey, @zellyn! How are you doing?

There is already a max-time flag. I guess you want it to cancel, as opposed to using deadline propagation? Could you instead achieve this outside of grpcurl? Like use & to run it in the background and then send the background process a SIGTERM after the timeout? I think that would achieve the same effect, without cluttering the already long list of flags for this thing.