Make it possible to submit and fetch supported connection types

[thelsing]

Is your feature request related to a problem? Please describe.
Currently a client has to chose if he wants to use webrtc or sockets to connect to a MT server. If the choice does not match with how the server is started the connection will fail after some timeout.

Describe the solution you'd like
The MT-Server should transmit all modes of connection it supports to the MT-registry. A client wanting to connect to it fetches this information, chooses the type of connection if there are more than one and connects.

Describe alternatives you've considered
Live with the status quo. But it will be more confusing if we get more options (HTTP, Websocket,...)