BBB as Videocall-Button

• support multiple rtc sessions (of different type) per room (js-sdk matrixRTCSessionManager.ts)
• decide if BBB is of type m.call or org.bigbluebutton.call (or session)
  • m.call -> interop with ec
  • org.bigbluebutton.call parallel call and BigBlueButton sessions are possible, we dont need to add encryption to BigBlueButton
• EW (Element Web) needs to support detecting and displaying different rtc session and a way to allow joining them
• Where do we store what widget needs to be used for which rtc session type
• Update the rust and js-sdk implementation to conform to the state key format from the spec (user_device_app) (not it is user_device)
• How does the join/creation flow look like (in EW)