jextract: pass arena from Swift instead of default interface for callbacks

[madsodgaard]

Previously, we generated default methods for the Java interface methods that require a SwiftArena. That was a way to basically use a global arena, instead of passing in one from Swift.

I think this PR is a better solution, where we pass in the global arena from Swift (a global cached ref) and not generate the default methods, unless the user has explicitly requested it through the allowGlobalArena memory management mode.

[ktoso]

Let's clean this up some more while at it, goal looks good tho