Backport "cache: Re-implememt cache on top of our external storage" into 1.16
This currently uses the ci_template.yml
from https://gitlab.freedesktop.org/nirbheek/gst-ci/-/commits/backport-re-implement-cache/. Will change it back once those changes are merged into gst-ci's 1.16 branch.
Also contains a commit from !434 (merged).