Adds a cleanup webhook called on disconnect and shutdown

[devinchristianson]

Adds a cleanup webhook called on disconnect and shutdown

Thanks for all of your hard work on this project - I'm enjoying it so much I've built a whole project around it to run labs/demos

This PR

• adds key cleanupserver with value HTTPClientConfiguration to the main app config
• converts the configuration to a webhook HTTP client on Backend Handler instantiation
• POSTs to the webhook OnDisconnect and OnShutdown with the connectionID, expecting a 200 response
  (note that it does not include the username because the Backend Handler does not have the username, presumably because not all backends have usernames, such as the SSH Proxy)

I've been using this code on my own project for a bit now and it's been working great!

Please let me know if you have any questions or would like me to make any changes to this PR as I'm happy to!

closes #486

---

By contributing to this repository, I agree to the contribution guidelines.

[codecov]

Codecov Report

Attention: Patch coverage is 18.51852% with 22 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (main@2e330b6). Click here to learn what that means.

Files	Patch %	Lines
internal/backend/handler.go	21.42%	10 Missing and 1 partial ⚠️
internal/backend/handler_factory.go	20.00%	7 Missing and 1 partial ⚠️
config/appconfig.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #567   +/-   ##
=======================================
  Coverage        ?   37.28%           
=======================================
  Files           ?      196           
  Lines           ?    17222           
  Branches        ?        0           
=======================================
  Hits            ?     6422           
  Misses          ?    10207           
  Partials        ?      593           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.