ci: reduce runner workload, add trigger, only run integration post-merge

The gstreamer-rs and gst-plugins-rs jobs are taking up a LOT of runner capacity.

Problems

• we run full post-merge pipelines - should cut this down to integration job only like in gstreamer monorepo
• every push to a user branch triggers a full build pipeline - should hide that behind a trigger like in monorepo (excl. rustfmt job perhaps), with Marge exception like in monorepo
• high windows runner load => probably don't need to run the msrv jobs on windows, make them manual

Todo

• move gstreamer-rs / gst-plugins-rs to a Marge-based work flow (Marge is already set up so should just work). I guess this will happen automatically once we add the trigger ;)
• Make windows msrv job manual (rationale is that 99% of what we want to check here is already checked by the Linux msrv jobs, so the extra value here is quite small)
  • !1132 (merged)
  • gst-plugins-rs!927 (merged)
• Combine windows features jobs so they all share some cached artifacts !1166 (merged)
• Add trigger job/stage, only activate automatically for Marge- see #401 (closed)
  • -> gst-plugins-rs!965 (merged)
  • -> !1155 (merged)
  • bonus points: rustfmt can always run just like gst-indent (and/or maybe all lint jobs?)
    • Missing for plugins-rs, its a bit more involved to do so
    • Works for gst-rs
  • only documentation job should run post-merge for main branch -> gst-plugins-rs!965 (merged)