Validate MultiLocation for asset-registry

[valentinfernandez1]

Improves the validation for the Multilocation received in register_reserve_asset inside the Asset-Registry pallet as discussed in #212.

[hbulgarini]

can we add one more tests to validate relay chain native token?

MultiLocation { parents: 1, interior: Here

[valentinfernandez1]

Using Here will not work since the check is based on how this is suggested by the XCM-Format.

chain/PalletInstance() for a Frame chain with a single-asset pallet instance (such as an instance of the Balances pallet).

In order for that to work it would need to be MultiLocation { parents: 1, interior: PalletInstance(4)} where the id 4 references the rococo pallet balances.

So I'm not sure whether we should tweak the validation or maybe I`m missing something. Including that is pretty simple but I just want to make sure that we are getting it right conceptually.

[hbulgarini]

But in PolkadotJS we are allowed to choose Here for the Concrete identifiers.

My guess is that it should work

[valentinfernandez1]

It's fixed with the commit I added yesterday.

        let (split_multilocation, last_junction) = location.clone().split_last_interior();

	let check = matches!(
		last_junction,
		Some(AccountId32 { .. }) |
			Some(AccountKey20 { .. }) |
			Some(PalletInstance(_)) |
			None
	);

If the Junctions is Here the function split_interior() will return None.

[hbulgarini]

looks great!