feat: idl @bit_bound or rust repr(...)

[carlocorradini]

Currently, DdsType ignores the #[repr(...)] attribute and determines the discriminant by simply counting the number of variants, rounding to the nearest representation. While this usually works, it fails in cases where a specific representation is explicitly defined.

For example, if we declare #[repr(u16)], DdsType will still generate a discriminator of type u8:

#[derive(DdsType)]
#[repr(u16)]
pub enum MyEnum {
    Value0 = 0,
    Value1,
    Value2,

This behavior is also reflected in the IDL generator, where we specify @bit_bound (no yet implemented).