Simplified foreign asset creator

[girazoki]

This PR proposes a pallet that is similar to the asset-manager from moonbeam but it is a simplified version of it. In this case, the foreign-asset-creator-pallet is only responsible for creating and maintaining an assetId<->ForeignAsset mapping to ease the following situations:

• knowing which assetId to touch whenever we receive an XCM MultiAsset
• knowing which MultiAsset to send when we are told to send a specific assetId to another chain

Important changes:

• We allow the creator to decide an assetId instead of deriving it ourselves through some other mechanism. The main reason for having a custom derivation for the assetId was that one could find a collision with a smart contract on the precompile address, but I believe this case is very ethereum specific and could be addressed in different manners (for instance, by using a smaller assetId type)

• No units per second, this if needed should be done in a different pallet

• No local asset creation

[girazoki]

Proposing it to moonkit instead!