webrtc: SessionDescription: access the inner SDPMessage as & or &mut

WebRTCSessionDescription owns its SDPMessage. The sdp() accessor used to return a copy of the SDPMessage which prevented the user from getting a ref and by extension from getting a mutable ref for in-place modification.

This commit makes the accessor return a reference to the inner SDPMessage and adds a mutable accessor.

Previous behaviour (getting an owned copy of the SDPMessage) is available by calling to_owned() on the reference returned by sdp().

Users who wish to change the type of WebRTCSessionDescription can call set_type().