Add advanced stats API

[JakobDev]

We currently have https://flathub.org/api/v2/stats/{appid}](https://flathub.org/api/v2/stats{appid} which provides a simple download count (sone install count) for apps. This is good for the Frontend and things like Badges, but it would be good to have a additional API which provides more information. A example of more information is the site https://klausenbusk.github.io/flathub-stats. You can switch between updates, installs and the sum of both. And you can see the stats split by Architecture. The data come from https://flathub.org/stats, which provides a JSON file with all Apps for each day. It would be nice, if there was some API to query this data for a App, so you don't have to parse hundreds of JSON files.

I also saw that https://flathub.org/stats provides a count for Countries and Flatpak version. Would be nice, if this data could added two.

The new API respond could be look like this:

• days
  • 1970.01.01
  • updates
    • architecture
      • x86_64: 42
    • countries
      • DE: 10
    • flatpak_versions:
      • 1.13.2: 5
    • total: 100
  • installs
    • same structure as updates
  • downloads
    • same structure as updates, sum of installs and updates
• total
  • same structure as per day

This new API could be used by 3rd party software to visualize all Data that Flathub has of a App. This informations would be interested for developer.

[adrianinsaval]

It would be nice to get stats for flathub-beta repo too