[Proposal] Add termination interface with success completion to runner

[kmrmt]

Is your feature request related to a problem? Please describe.:

Current Runner interface doesn't provide success completion.
In some cases, I want to terminate Runner with success.

Describe the solution you'd like:

Runner#Start returns (context.Context, <-chan error, error) and cancel in Start if needed.
And then, cancel parent context when catching children's context cancellation.
It can terminate process with success completion.

Describe alternatives you've considered:

Now I send SIGTERM to itself. I think it is worth than above.

[rinx]

For agent sidecar on init container mode, it will be also useful.

[kpango]

I'll work on it, and this Issue is too old.

I'll close this issue now, but I'll refer this Issue after new PR has been created.