sdp: support multiple rid parameters

sdp: support multiple rid parameters

As specified formally in RFC8851

Each rid description is placed in its own caps field in the structure.
This is very similar to the already existing extmap-$id sdp<->caps
transformations that already exists.

The mapping is as follows:

  a=rid:0 direction ';'-separated params

where direction is either 'send' or 'recv'

gets put into a caps structure like so:

   rid-0=(string)<"direction","param1","param2",etc>

If there are no rid parameters then the caps structure is generated to
only contain the direction as a single string like:

   rid-0=(string)direction

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1760>