3rd part of the change started by D28882 and D28884.
Instead of implementing KWayland::Server::RemoteAccess, we implement
KWayland::Server::Screencasting and deal with it with ScreenCastStream
that used to be in xdg-desktop-portal.
This allows us to offer video streams directly from kwin rather than
building them from the outside (be it xdg-desktop-portal or otherwise).
I think we can drop support for PipeWire 0.2 (you don't seem to search for it anyway) so you can drop all PW_CHECK_VERSION(0, 2, 90) and keep just the branch for PipeWire 0.3. I will do some proper testing tomorrow.
I have tested this and it now works as before, tested with Chromium, while checking all the values we pass to the portal and PipeWire which seem to be correct.