Extract process killing from custom executor

What does this MR do?

Extract process killing from the custom executor

Why was this MR needed?

The custom executor has the ideal situation where we want to first send SIGTERM to the process to allow the process to clean up. If the process is still alive after a specific deadline it will send a SIGKILL as explained in https://docs.gitlab.com/runner/executors/custom.html#terminating-and-killing-executables

Extract this logic to it's package so it can be reused for executors like the shell executor to solve issues like #3376 (closed)

👋 When reviewing this merge request take a look at !1551 (closed) 👋

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

• Broken out of !1551 (closed)
• Part of #3376 (closed)
• Part of #4438 (closed)