Support custom keys with `appcenter::` prefix

[danirabbit]

Problem

It looks like the recommendation in appstream changed from using x-vendor prefix to vender:: prefix

Proposal

Make sure we support appcenter::stripe-key and appcenter::suggested-price as values in the custom tag

Prior Art (Optional)

https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-custom