[8.16](backport #41960) Add upstream deadlock warning to the logstash output

[mergify]

Add an internal timeout in the Logstash output so that it logs an error when published events stop making progress for an extended time, which can indicate that the Logstash host is silently deadlocked.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related Issues

• Resolves [libbeat][outputs/logstash] - The logstash output waits for acknowledgements forever #41534

---

This is an automatic backport of pull request #41960 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 0e62bf8 has failed:

On branch mergify/bp/8.16/pr-41960
Your branch is up to date with 'origin/8.16'.

You are currently cherry-picking commit 0e62bf8f0.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   libbeat/outputs/logstash/deadlock.go
	new file:   libbeat/outputs/logstash/deadlock_test.go
	modified:   libbeat/outputs/logstash/sync.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   libbeat/outputs/logstash/async.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[botelastic]

This pull request doesn't have a Team:<team> label.

[mergify]

This pull request has not been merged yet. Could you please review and merge it @faec? 🙏