Webrtc: fix RTCSessionDescription

[havalli]

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What does this PR do and why is it necessary?

Fixed the connection to the webrtc server.

How was it tested? How can it be tested by the reviewer?

Tested with a mediamtx Webrtc server
Webcam url: "webrtcs://webrtc-server-host/cam/whep"

Any background context you want to provide?

What are the relevant tickets if any?

Screenshots (if appropriate)

Further notes

[cp2004]

The (incomplete) WebRTC implementation in OctoPrint is only compatible with the aiortc server it was developed for (by another contributor). This change will probably break that and any similar servers that communicate the same way.

Because there is no standard to the WebRTC offer communication, these should use a Webcam Plugin introduced in 1.9.0, which allows you to customise the connection to whichever server you are using. I've written one for camera-streamer.

[havalli]

There is a standard for the WebRTC offer information. Whep. Yes, it is still a draft. But it fits this scenario exactly. (Just consuming a video stream)
My changes follow this standard. Wouldn't it make sense to put the existing logic in an "aiortc" webcam plugin and implement the Whep standard in the Classic Webcam?

[foosel]

Are these changes all that is needed to make it Whep compliant? If so I would suggest to add a config option to the plugin, something like negotiate_flavor, and depending on whether this is set to aiortc or whep do one or the other thing.

[havalli]

In my setup, it works. I think it's more of a starting point. Whep is also still a draft. I like the configuration option, but my Python skills are very bad. Maybe someone else can implement it.

[foosel]

Converting this to draft for now, as we cannot merge it as is. Requires a negotiate_flavor configuration option from the current point of view, in order to not break compatibility with existing aiortc based setups.