Add key conversion mod support for osu!mania beatmaps

[DaiterGG]

added support for mania to mania conversion
fist time contributing so unsure about pretty much everything, but mainly the way I added ApplyToBeatmap() method

Closes #18562

[LeNitrous]

I don't think this check is needed since there is no valid case this mod is ever used outside of mania.

[DaiterGG]

This mod convert std maps to different key counts.
And this conversion shouldn't apply for those

[DaiterGG]

It can be removed since all checks later in this method will fail

[bdach]

Structurally none of this works as far as I'm concerned. The code performing the conversion should be in ManiaBeatmapConverter. Because this PR isn't putting it there, ManiaModDualStages is probably going to break (and if it were to be implemented similarly, there'd be code duplication and/or an introduced order dependency). The IsForCurrentRuleset conditional in the keymod should be gone because the beatmap converter should be handling it. And so on and so forth.

Not even looking at the implementation details because this just can't work structurally as is.

[DaiterGG]

I took this approach because it's just easier for me.
Gonna look at different way now, thank you

[bdach]

Just as a reference, I'd like the current conversion algorithm from osu! to mania to be clearly delineated from the changing of key count in mania maps specifically, something like so:

if (isForCurrentRuleset && TargetColumns != columns)
{
   // this PR goes here
}
else
{
    // existing conversion code goes here
}

[DaiterGG]

9e4e3c93-0be7-49d6-944e-6bc515a28b55.mp4

here is some visuals for new algorithm
it preserves more original structure where possible without deleting more notes