`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()`.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1406>
These assertions can only trigger because of bugs in the bindings
implementation or in the C code and not because of bugs in calling code,
so using debug assertions is perfectly fine for them and reduces the
number of assertions inlined everywhere in release builds.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1188>
