Add support to transfer_assets() call/extrinsic of pallet XCM

[IkerAlus]

paritytech/polkadot-sdk#2388 introduced a new call to pallet XCM. This call is a sort of universal interface for XCM transfers:

• First, it is supposed to take care of (most of) transfer cases internally, avoiding the user to think when they should do a teleport or a reserve backed transfer and hence making the mentioned calls unnecesary.
• It also take cares of more complex scenarios as for example teleporting foreing assets to Asset Hub but paying fees with DOTs (reserve backed transferred).
• Finally, it also enables the bridging tokens back to Ethereum.

Particularly because of the two latter points above, Asset Transfer API should add support to this transfer and enable it when it connect to runtimes exposing it.

This PR was included in Polkadot-SDK v1.5.0.

As of creating this issue, this call is already live on Westend and Rococo testnets.