Add image caching

[5Avian]

The entire reason for making this application server-side was to make it possible to embed the generated image directly into mod descriptions without having to download & upload the image. Currently every request causes a Modrinth API call, a Modrinth CDN download and the creation of a new image. By adding caching, we can reduce API calls and image operations.

The following things could be cached:

• Project JSON objects (only necessary values) (updates after x amount of time)
• Mod icons (updates when the icon url in the project changes)
• Generated badge images (updates when the icon changes)

Besides these conditions, the cache should also be limited to a configurable amount of bytes. This would mean the least used images/icons should be dropped from the cache when it's full.