[Enhancement] Cosmetica proxy

[Madis0]

Besides providing their own capes, Cosmetica also has a cape proxy that connects to other cape providers.
This has several pros and cons:

Pros

• Less requests/bandwidth use
• More privacy as the users do not connect to providers directly and can fetch all capes over HTTPS
• Providers that may otherwise be inaccessible (e.g. Lunar)
• Serverside URL changes in case a provider changed their API (Cape shows as "UPDATE YOUR MOD" despite having no new version #118)

Cons

• Cape uploads take more time to get reflected in the API
• Providers can be toggled, but are limited to Cosmetica's options (though the mod could use some other providers with a separate request)
• All providers would go down if Cosmetica's site does
• Technically a proxy is a black box that can modify what's displayed

Hence I suggest adding an option to use Cosmetica proxy for the providers.

Existing implementation: https://github.com/Cosmetica-cc/Cosmetica