webrtcbin: Expose SCTP Transport

Being able to access the SCTP Transport object from the application means the application can access the associated DTLS Transport object and its ICE Transport object. This means we can observe the ICE state also for a data-channel-only session. The collated ice-connection-state on webrtcbin only includes the ICE Transport objects that resides on the RTP transceivers (which is exactly how it is specified in https://w3c.github.io/webrtc-pc/#rtciceconnectionstate-enum).

For the consent freshness functionality (RFC 7675) to work the ICE state must be accessible and consequently the SCTP transport must be accessible for enabling consent freshness checking for a data-channel-only session.