XCM: Make locking work with multiple assets

[gavofyork]

Currently pallet_xcm only supports the lockability for a single asset via the Currency and CurrencyMatcher types. A new trait should be introduced (perhaps AssetLocking) which is similar to TransactAsset except but with can_lock_asset(what: &MultiAsset, who: &MultiLocation) -> XcmResult, set_lock_asset(what: &MultiAsset, who: &MultiLocation) -> XcmResult and extend_lock_asset(what: &MultiAsset, who: &MultiLocation) -> XcmResult.