Draft: whipsink: several fixes + improved support for ICE servers
Depends on gstreamer!3217 (merged), do not merge before that.
Improves compatibility of the whipsink plugin with the standard, by letting it use the ICE servers returned in the POST request, among other fixes.
See commit messages for details.
The last commit removes the OPTIONS request altogether, see reasoning in the commit message. This assumes gst-plugins-rs is released together with gst-plugins-bad; if this is not the case (or for any other reason you prefer otherwise) the last commit can be removed or put into a separate MR.