feat: Support for DTLS Connection ID (CID)

[HMBSbige]

Would it be possible to add Connection ID (CID) support?

• DTLS 1.2: RFC 9146
• DTLS 1.3: RFC 9147 (the C bit in the unified header)

CID allows demultiplexing by connection identifier instead of the 4-tuple, which is useful for NAT rebinding and connection migration scenarios.

Currently the implementation explicitly discards records with the CID bit set.

[HMBSbige]

Is the reason CID is not implemented that ICE already handles address changes in WebRTC scenarios?

[algesten]

I don't mind if we support it.

In WebRTC we have "ice restart", which is a quicker state resumption used in this scenario. It does however tear the entire DTLS stack down and starts over.