Integrate ORML `xtokens` into parachain runtime

[vayesy]

As a participant of substrate ecosystem, I would like to transfer fungible assets between zero and other parachains, which are connected to the same relay chain.

How to implement

Following steps from the Acala docs page, provide CurrencyIdConvert type with possibility to convert between zero currencies and other parachains currencies.
Extend this type to be able to convert CurrencyId to MultiLocation and vice versa.
Add xtokens pallet to parachain runtime, providing above types as pallet parameters.

How to test

1. Setup Zero and Acala parachains connected to the same relay.
2. Open HRMP channel between two parachains.
3. Initiate aUSD token transfer from Zero to Acala. Make sure tokens transferred and received on Acala side.
4. Repeat previous step the other way around.