exp: implement internal queue.kill support

[pmrowla]

dvc-task/exp queue currently support experiments.queue.remove for removing inactive experiments. experiments.queue.kill API call for cancelling/terminating individual active experiments is still needed.

prerequisite for #7591

Job termination requires:

• Terminate exp run subprocess via dvc-task proc.kill
• Celery tasks related to the running experiment should not be revoked, they should run to completion
  • Once the exp run child process is terminated, it should be treated as any other failed run (and the relevant cleanup/partial collection celery tasks should still be scheduled and run)

[karajan1001]

There are still some related works

1. unit test code coverage for the kill method.
2. implement the kill flag in shutdown.

[karajan1001]

close due to #7714 and #7799