Expose runtime for apps in the API

[ahayzen]

Rational

We would like the know the number of apps that use a particular runtime (eg how many have transitioned to freedesktop 18.08). If the flathub api exposed the runtime and runtime version as a property in https://flathub.org/api/v1/apps/ then we could generate such charts.

Proposal

Add "runtime" and "runtime-version" fields to each app in https://flathub.org/api/v1/apps/ which could have the values "org.freedesktop.Platform" and "18.08".