Add support for "freestyle" in multiplayer

[smoogipoo]

Prereqs:

• Add support for "freestyle" in multiplayer osu-web#11760
• Add support for "freestyle" in multiplayer osu-server-spectator#251

Preamble

I was told to effectively hack this in, and was not given an opportunity to refactor multiplayer to properly support it or even to be able to write tests without pulling my hair out. So here goes.

Have I done some basic testing to see things work? Yes.
Will things break? Definitely.
Do I know what will break? Nope.

Description

2024-12-24.19-46-05.mp4

I've named this "freestyle", because I see a future where this is more than just beatmap/ruleset but also mods and thus include "freemods" in it. Perhaps something like a menu where you can choose what is free.

For the time being, when it's enabled, it will disable mods/freemods. I'm not sure what the intended UX is, but it will definitely be a lot more complicated due to mod types not matching between rulesets, i.e. this will fail a ton of validation everywhere. So I'm keeping it simple.

There's no display in the room for what/how other users have chosen to play, I'm not even sure where I'd add that in because the room design is a whole bunch of mess. There's already no display of other users' freemods either, so /shrug?

[peppy]

There's already no display of other users' freemods either, so /shrug?

It's shown here. I think adding the ruleset icon in the same flow area at minimum would be good to have.

[peppy]

@ppy/team-design able to provide some basic design direction for how ruleset and difficulty selection could be shown per-user here?