Add Asset Rate pallet

[stiiifff]

The FRAME Asset Rate pallet provides means of setting conversion rates for some assets to the chain’s native token.

Trappist XCM configuration currently contains some hard-coded conversion rates for Rockmine USD (Rusd) & Rococo’s ROC token to Trappist’s native token (HOP).

We should factor out this logic and leverage the Asset Rate pallet functionalities instead (which also allows to change the rates at a later stage). Those conversation rates will be potentially used in a number of places (XCM operations, or local asset swap operations).

[metricaez]

I can jump into this, already had to manages and create Traders for #248