Add online-mode Cargo feature

[mat-1]

Closes #280.

Needs testing to make sure it actually works properly and doesn't break plugins.

Would've been nice to completely disable azalea-auth when this feature is disabled, but currently GameProfile and offline::generate_uuid are in azalea-auth. Maybe they could be moved to azalea-core and azalea-crypto later?

[aparatext]

Wouldn't it also disable the AES/RSA dependencies? Offline mode uses plaintext transport I think.

[mat-1]

As of a few versions ago, offline-mode servers have the option to enable encryption (not sure why tbh), so the encryption-related dependencies are unfortunately still required.

I think rsa dependency can be removed since it's only used for chat signing though, so thanks for pointing that out. :)

[mat-1]

Would've been nice to completely disable azalea-auth when this feature is disabled, but currently GameProfile and offline::generate_uuid are in azalea-auth. Maybe they could be moved to azalea-core and azalea-crypto later?

I moved the offline module to azalea-crypto in ba3a883.

Moving GameProfile out of azalea-auth is trickier though, because moving it into azalea-core would add a relatively heavy dependency to azalea-auth (which is often used standalone).

One solution would be to significantly trim down azalea-core or add tons of feature flags, but this would just shift the problem I'm trying to solve (having lots of #[cfg(feature="...")] attrs). Another solution would be to make a new crate to put GameProfile in, but an entire crate just for that type doesn't feel like it would make sense. Not sure what the right solution here is.