XCM: Introduce pallet_assets::Config::CreateOrigin

[gavofyork]

pallet_uniques already has an item CreateOrigin in its Config trait. The equivalent item should be introduced into the Assets pallet.

It should look basically like this:

/// Standard class creation is only allowed if the origin attempting it and the class are
/// in this set.
type CreateOrigin: EnsureOriginWithArg<
	Success = Self::AccountId,
	Self::Origin,
	Self::ClassId,
>;

fn create should be altered to respect it.