rtsp-server: Adds method to set the RTSPToken on some RTSPContext
This change adds a method to set the token on an RTSPContext, and increase its refcount.
This is to address a problem I encountered and reported in #1399 (closed), whereby it isn't possible for Python to directly set the token in the context, nor increase the refcount.
This may or may not be the right way to do it, but I've tested it and it now allows me to set tokens correctly in a Python-based GStreamer RTSP Server