Support getting stream_id and track_id of Media

[uwuPyxl]

Get the stream_id and track_id from the incoming MediaLine and expose them to users on the resulting Media so they can tell incoming Media apart. Should the incoming Media not have a Msid attribute this falls back to the previous behavior of generating random ids

[algesten]

Hey! Thanks for this!

I was under the impression that msid was deprecated, but I think I'm mistaken. The a=ssrc lines are not in fashion anymore, which means getting msid there is not a good idea. But this PR is working with a=msid, which seems to still be good API. Is that your understanding too?

[uwuPyxl]

I just used the msid attribute since it was most obvious and convenient but yeah it does seem to be intended for this purpose.

[algesten]

Ok. Let's merge, but 3 small formatting things first.

[uwuPyxl]

I've updated the formatting manually, but it should be possible to set up a rustfmt.toml to configure the formatting rules