Assets registry should also hold reserve locations

[acatangiu]

Assets can have multiple reserves, generally origin chain + AssetHub. Which chain can act as a reserve is important when trying to build more complex transfers.

This becomes especially important for bridged assets where their origin chain is not directly accessible, and the chain bridging them in acts as a reserve.

This information will be required for more complex asset transfers support paritytech/asset-transfer-api#400

There was also more discussion on this topic here: https://forum.polkadot.network/t/xcm-asset-registry/1168/3?u=acatangiu