v21.0.0 Release Todos

To be done not earlier than just before releasing v21.0.0 stable, or after there was a devXX bump.

* [x] Remove `ClientControlPacket::VideoErrorReport`
* [ ] Merge ControlSocket with StreamSocket
* [ ] Refactor packet protocol
* [ ] Make prefix data little-endian
* [ ] Make packet length contain itself (add 4 bytes to the count)
* [ ] Remove extra 4 bytes to the max shard length
* [x] Send combined eye gaze with separate field
* [x] Send eyes already in local head space.
* ~~[ ] Use a json string in place of the `VideoStreamingCapabilities` packet.~~ Using ext_str instead
* [x] Make multimodal input protocol default
* ~~[ ] Send velocity for skeletal hand tracking~~ decided not to do/not needed
* ~~[ ] Use mesh foveated rendering~~ deferred. Needs protocol support to negotiate disabling FFE.
* ~~[ ] Add static controller offsets on the client and make parameter exposed by dashboard default to [0,0,0]~~ was done already in v20
* [ ] Include `DecoderConfig` packet in the video frame header, to avoid having to request another IDR and resend the `DecoderConfig` two times.
* [x] Make ~~all~~ some stream header packets extensible with a ~~`Vec` (don't use json)~~ String decoded with json. Values can only be appended and not removed. ~~Alternatively investigate CapnProto or Flatbuffers which support protocol extensions natively.~~ overkill
* [x] Use mutually exclusive tracking sources (e.g. 2 eyes or combined gaze, fb or htc face tracking, fb or pico body tracking), and wrap it with Option.
* [x] Remove limited range encoding support
* [ ] Change dashboard API to not require a body for requests. This aligns better with the ureq 3.0 API, and standard HTTP practices
* [ ] Reorder settings: create SteamVR tab, rename headset to Tracking, for each tracking method, put source settings in Tracking and sink in SteamVR or Extra->Sinks (for example OSC/UDP/VMC)
* [ ] Switch to bincode 2.0