Adds an option `--all` to unregister command

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What does this MR do?

adds an option --all to unregister all runners. fixes #2500 (closed)

Why was this MR needed?

In order to unregister all the runners on the given machine. It can be useful when your CI runners are ephemeral, and can die at any time.

Are there points in the code the reviewer needs to double check?

One thing I'm unsure about is how active runners will behave when jobs are in progress.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Closes #2500 (closed)