build: Move gvdb to a subproject

Rather than carrying the copylib around inside GLib, which is a pain to
synchronise and affects our code coverage statistics.

This requires updating the CI images to cache the new subproject,
including updating the `cache-subprojects.sh` script to pull in git
submodules.

It also requires adding `gioenumtypes_dep` to be added to the
dependencies list of `libgio`, since it needs to be build before GVDB as
it’s pulled in by the GIO headers which GVDB includes.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #2603

docs/reference/gio/meson.build

@@ -4,7 +4,6 @@ if get_option('gtk_doc')
 
   ignore_headers = [
     'gdbus-2.0',
-    'gvdb',
     'inotify',
     'kqueue',
     'libasyncns',