Allow blinded fees in transactions

Sniped from @kallewoof :

Without blinded fees,

* If both asset type and value are visible, roughly 50% of all asset types will be in plain view of listeners; not just that, but they will also deduce that the given asset is the source type, as fees are generally paid using the source asset to obtain the target asset.
* If only value is visible, this is basically fine, as a lot of transactions will have close to the same fee, but if the asset types have distinct values on the chain, the observer will be able to deduce the type of asset (worst case) or a list of potential asset types based on tx size vs fee value.*

An alternative which does NOT need this would be to have a fee asset which is always used for fees; the transaction would then have to include a fee input. This may have some properties that business side would enjoy, as it lets the fee issuer(s) control the total chain velocity (is that even an expression?).

(* For Japan, most "general type" points have a 1-to-1 ratio with the yen, but there are some companies which have e.g. 1 point = 5 yen. The latter type would suffer from having value visible above, unfortunately.)