extensions-app: Support "version-name" metadata field

extensions-app: Support "version-name" metadata field

The extensions site recently added support for a custom
"version-name" string in metadata:
gitlab.gnome.org/Infrastructure/extensions-web/-/merge_requests/154

This allows developers to control the version that is exposed to
users. As the version according to the developer is almost always
more relevant than the automatic version assigned by the website,
use it instead of the "version" field if set.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2995>
(cherry picked from commit 6db55eaea6bb1941d0e1e40d026ab3da7d41977c)