Buffer update feeds to disk after download

[kyu-n]

When starting the launcher, we pull the main & update feeds to verify game file integrity. Then, after we start the client, we pull them again to get critical server information (login/etc.)

While this doesn't normally matter, slow internet connections (e.g. satellite) have a worse experience due to the redundant pull. We can reduce startup times significantly by caching the update feeds to a cache dir and hinting to the client that it should look there before pulling them down.

Tasks:

• Client implementation (hint parameters, feed verification)
• Launcher impl

Questions:

• What directory would be most suited for this? $XDG_CACHE_HOME?
• Any concerns between flatpak / snapcraft / default access to said cache directories?