Add option to set a stun server for RTSPtoWebRTC

[allenporter]

Proposed change

Add a configuration option to set a stun server for RTSPtoWebRTC, and a websocket exposed so that the javascript frontend webrtc player can fetch it.

Frontend PR: home-assistant/frontend#13942

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• [X} There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

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• Documentation added/updated for www.home-assistant.io

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• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
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[allenporter]

Converting to draft as I need to add translations.

[balloob]

Can be merged if frontend pR is ready

[github-actions]

There hasn't been any activity on this pull request recently. This pull request has been automatically marked as stale because of that and will be closed if no further activity occurs within 7 days.
Thank you for your contributions.

[allenporter]

I've sent a new version of a frontend PR that uses this in home-assistant/frontend#12084

[allenporter]

I've made a few updates to the config flow. This can use another look, though frontend PR that depends on this has not yet been reviewed.

[allenporter]

The frontend PR has been approved: home-assistant/frontend#13942 and this is ready for another review after some updates/test improvements related to the options flow.