Add FreeDesktop Appstream Metadata spec

[Tbusk]

Feature request

Description

Add appstream metadata file to the repository for the UI to have a consistent app-store experience on linux.

Describe the solution you'd like

Create a metainfo xml file following specs from freedesktop so downstream implementations don't need to make their own metainfo file, and can have the same experience among listings in app stores.

[Tbusk]

This is one I currently am using. It would be nice to get one up-stream though.

https://github.com/getsolus/packages/blob/main/packages/i/ivpn/files/ivpn.metainfo.xml