screen-cast-stream: Embed the wayland output name to streams

If a screencast session is screencasting from multiple monitors,
it is not possible to distinguish which stream (or PipeWire stream
node id) belongs to which connected monitor (such information may be useful
to the caller e.g. caller might want to embed the stream in a window
and name it after the monitor connector, for example). This change adds
optional metadata for monitor streams to also return the wayland output
name with the stream.

Ideally, this metadata should equip the caller to get more information about
display from the following Wayland interfaces:

- GNOME/xdg-desktop-portal-gnome!48
16 jobs for !2540 with embed_wayland_output_name in 36 minutes and 31 seconds
merge request