Add no-clients-timeout option

[tmi]

Closes https://github.com/dask/distributed/issues/7959 .

I aimed for a minimalistic implementation, and named it consistently with the existing no_workers_timeout.

• The original issue does not address whether to respect running/queued jobs or not, I chose to do so as it made more sense to me,
• I consider activity from both clients and jupyter app, though not sure if the latter is explicitly necessary,
• I do not support "state transition" (like the existing check_idle does) as it didn't seem necessary.

I've added a basic test -- though it's rather crude, I'm happy to accept a suggestion how to do it differently.

• [ ] Tests added / passed
• [ ] Passes pre-commit run --all-files

[GPUtester]

Can one of the admins verify this patch?

Admins can comment ok to test to allow this one PR to run or add to allowlist to allow all future PRs from the same author to run.

[github-actions[bot]]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

    29 files  ± 0      29 suites  ±0   11h 26m 33s :stopwatch: + 8m 2s  4 084 tests + 1   3 968 :white_check_mark: + 5    112 :zzz: ±0  4 :x:  - 4  55 244 runs  +14  52 805 :white_check_mark: +20  2 434 :zzz: ±0  5 :x:  - 6 

For more details on these failures, see this check.

Results for commit 6a6b4454. ± Comparison against base commit 8564dc79.

:recycle: This comment has been updated with latest results.