Support custom keys with `appcenter::` prefix

### 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