Add attribute keep_case for NifUnitEnum

NifUnitEnum currently expects atoms corresponding to struct fields to be in snake case. This works fine when the user can define the atoms to be in snake case. If that is not possible, e.g. because they are defined by a third party, then decoding them with NifUnitEnum is not possible.

I suggest we add an attribute keep_case which defines that the fields are to be used exactly as-is as atoms.

#[derive(NifUnitEnum)]
#[rustler(keep_case)]
pub struct KeepCase {
  UPPERCASE,
  lowercase
}

Start in rustler_codegen/src/unit_enum.rs around the snake-case conversion referenced by the issue, and trace how NifUnitEnum attributes are parsed. Done means the keep_case attribute allows enum fields such as UPPERCASE and lowercase to be used exactly as atom names, while existing behavior remains unchanged.