[prism] Add an idle shutdown timout to prism binary.

[lostluck]

Adding a flag --idle_shutdown_timeout to avoid SDKs leaving dangling prism processes after a job terminates.

• Operates simply: Tracks the number of terminations, and compares to the number of started jobs. If they're equal, then start the idle timer.
• Timer is cleared if there's a new job.
• Initial timer set on start of prism process.
• Currently only actively running jobs extend the timeout, and other actions (eg. via Job management) do not extend the timeout.
• Negative durations do not idle shutdown.

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Mention the appropriate issue in your description (for example: addresses #123), if applicable. This will automatically add a link to the pull request in the issue. If you would like the issue to automatically close on merging the pull request, comment fixes #<ISSUE NUMBER> instead.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

To check the build health, please visit https://github.com/apache/beam/blob/master/.test-infra/BUILD_STATUS.md

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI or the workflows README to see a list of phrases to trigger workflows.

[lostluck]

R: @jrmccluskey

[lostluck]

This will end up being cherry picked into the release, along with an associated change to use them with the Java and Python runner wrappers.

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control. If you'd like to restart, comment assign set of reviewers

[jrmccluskey]

LGTM, one docstring change

[lostluck]

Thank you very much!