Add the appropriate changes to make Legacy clients authenticate properly when a custom server is selected

[Midou36O]

The commit effectively rewrites portions of the onlinefix java classes to support custom domains through flags, and adds the flags when a custom auth account is being used. This allows the legacy minecraft java versions to be able to pull skins and join (I did not test however) servers.
Also switches OnlineFixes from the default false to true

[evan-goode]

Oh did you see my branch https://github.com/unmojang/FjordLauncher/tree/evan-goode/legacy? I linked to it in one of the Matrix rooms but did not open a PR yet, so it was not that visible, sorry.

Your patchset looks like it does the job also, but I made some additional changes to get it working with any authlib-injector API server, not just Drasl. Fjord must also support Ely.by, Blessing SKin, etc. The authlib-injector spec is notably missing /game/joinserver.jsp, and https://wiki.vg/Mojang_API#Username_to_UUID, so we have to work around that.

[Midou36O]

Oh whoops, I didn't see that branch at all, i thought it was just some sort of theory and no actual code was written yet, my bad.

Yeah I mostly focused on mojang auth, but looks like you're always 2 steps ahead than me 😄
I'll close this PR in favor of the legacy branch work then, will make sure to make reviews once the PR is available.