cache results of SB requests

[kavorite]

See MegaAntiCheat/client-backend#159. If we're to use https://steamhistory.net/api, we have to centralize the cache for it. As of now, this is the only place to do that.

Edit: Add acceptance criteria.

• Cached /sourcebans endpoint that returns SH API JSON payloads
• (enhancement) persist the cache in the database
• (enhancement) return response bodies augmented with times to live for cached results

[lili7h]

Updated my plan to remove the points that get solved by steamhistory and masterbase caching

[kavorite]

As per MegaAntiCheat/client-backend#159 (comment), no longer strictly necessary