support MultiLocation fungible assets with XCM transfers

[brenzi]

Requirement:

• Any fungible asset in the same relaychain ecosystem can be transferred to the encointer parachain, including all fungible assets on Statemine

Acceptance:

• Manual Rococo-local setup that successfully transfers
  • a native token from an alien parachain (i.e. Integritee)
  • a custom fungible asset on a Statemine instance
    and queries the balance on encointer

The asset hub supports this already, so we may be able to copy-paste.
However, the asset hub only supports teleport. So it assumes that all other parachains accept asset hub as IsTeleporter destination. Encointer is teleporter to the relay chain, like the asset hub. but I'm not sure it makes sense to require teleport. A reserve-backed transfer would probably be easier as it doesn't require a runtime change of the other parachain

Alternatively: We could open a single HRMP channel to asset hub and tell everyone to move their tokens over that route.