webrtcbin: fix msid line and allow customization

From https://datatracker.ietf.org/doc/html/draft-ietf-mmusic-msid-16:

Multiple media descriptions with the same value for msid-id and msid-appdata are not permitted.

Our previous implementation of simply using the CNAME as the msid identifier and the name of the transceiver as the msid appdata was misguided and incorrect, and created issues when bundling multiple video streams together: the ontrack event was emitted with the same streams for the two bundled medias, at least in Firefox.

Instead, use the transceiver name as the identifier, and expose a msid-appdata property on transceivers to allow for further customization by the application. When the property is not set, msid-appdata can be left empty as it is specified as optional.