[SCTP] Expose MTU in `sctp::association::Config`

[oteffahi]

Hello,

Is there a reason why the MTU is not configurable on the webrtc-sctp crate?

Correct me if I'm wrong, but the crate and its supported connections don't seem to implement PMTUD, and therefore the connection remains stuck on the default INITIAL_MTU=1228. I understand that the currently hardcoded value works for WebRTC purposes, but as it stands it is just too small for our uses-cases.

it also seems that the Pion implementation exposes this: https://github.com/pion/sctp/blob/b1a66a4b0fe20254da90495abf4013feac6815fd/association.go#L275

Thank you for your time.