Add a way to cancel commands

[khanhtc1202]

What would you like to be added:

We can cancel deployment via the deployment page on the PipeCD console, but we don't have any way to cancel other kinds of commands to piped effectively. Up to now, mostly we can just wait for those commands to be marked as orphan or timeout, but in some cases, users may need a way to cancel those commands (planpreview command for example)

This issue is more like an open for discuss/opinions, and could be convert to discussion later.

Why is this needed: