Pion ignores the ice-ufrag of candidates

[jech]

I've had this on my todo list for a while, but now that @JoeTurki seems to be cleaning up the code, it's a good time to write it up.

At https://w3c.github.io/webrtc-pc/#dom-peerconnection-addicecandidate, there's the following language:

5. If candidate.usernameFragment is not null, and is not equal to any username fragment present in the corresponding media description of an applied remote description, return a promise rejected with a newly created OperationError.

This requirement ensures that a remote candidate that was sent before renegotiation but arrives after a renegotiation is not erroneously applied to the wrong session. An example of a protocol that can cause such reordering is WHIP.

As far as I can tell, Pion does not obey this requirement, the ufrag is simply ignored.

[JoeTurki]

I was able to make a test to replicate the condition for this bug to happen with WHIP, and I will work on it after #2990 is merged :)