Support for references & population with Ref<M: Model> enum

[jonahseguin]

pub enum Ref<M: Model> {
  Id(Id<M>),
  Model(M),
}

allow population with #[musty(mongo(populate))] field attributes, and possibly generate QueryOptions for each model, to allow for usage like so:

#[model(mongo)]
#[derive(Filter)]
struct User {
  id: Id<Self, u32>,
  address: Ref<Address>,
}

#[model(mongo)]
struct Address {
  id: Id<Self, u32>,
  street: String,
  city: String,
  country: String,
}

let user = User::get(
  &db, 
  User::filter().id().eq(1).build(), 
  User::options().address().populate(true).build()
).await?;

[jonahseguin]

References done, population next