Support for specifying STUN server: input validation (1/3)

[jotaen4tinypilot]

Related https://github.com/tiny-pilot/tinypilot/issues/1460.

This PR adds the backend validation logic for processing the user-provided input values for the STUN server (server + port). These values will later come from the UI.

Notes

• I added comments in the janus.jcfg file to shed some more light on how the config parameters work.
• Based on these rules, I implemented the request validators.
  • I created a unified, public parse_h264_stun_address validation, to enforce that they are either both present or both absent.
  • The _SERVER_PATTERN is intentionally broad, I’m not sure it would be worth to have a stricter validation here. (At least I couldn’t find any reasonably simple way, without either making the regex more complex, or pulling in an external library.) For our purposes, it’s probably enough to provide a sanity check, rather than a bullet-proof validation.
  • The IP address validation can be done conveniently via the ipaddress native Python package.

[jotaen4tinypilot]

Automated comment from CodeApprove ➜

⏳ @jdeanwallace please review this Pull Request