Fill in sdpMid for local candidates

[youmustrust]

Summary

ICECandidate generates an empty sdpMid for local candidates. If the remote peer doesn't use sdpMLineIndex for locating a media description, all local candidates generated by Pion would be dropped, possibly leading to connectivity check timeout.

Motivation

N/A

Describe alternatives you've considered

N/A

Additional context

aiortc uses sdpMid alone for locating a media description. Since they don't support peer-reflexive candidates, connectivity check timeout occurs with Pion. For this issue, I opened this PR to make aiortc respect sdpMLineIndex.

[Sean-Der]

Hey @youmustrust

Sorry for the delay on this, been tackling other issues. aiortc + Pion are able to interop, we do testing on webrtc-echoes

[youmustrust]

@Sean-Der Oh, sorry. This occurs when ICE lite is enabled.