[v1] Add Fee Policy

[tiero]

Once we have #670 implemented, we need to add possibility to the operator when creating the market, to specify the fee policy ie. allowing the trader to select the fee asset he will be charged for the trade, either allowing both assets or only one (in case of a security asset we cant use that asset as fee)

[altafan]

One way to implement this would be adding a new map field to the market domain AllowedFeeAsset map[string]struct{} to be used when serving a preview or proposal request for making the required checks over the fee asset presented by the user.

[tiero]

one asset is alwats mandatory, max 2