Add fee property to validation of swaps `/trades` response

Comment from @Gudahtt 

> I just noticed that the fee is missing from the quote validator. We should add the fee to that at some point.

> We might also want to consider filtering the API response to just the validated parameters, so that we're forced to consider this in the future as the API changes.

https://github.com/MetaMask/metamask-extension/pull/9677#pullrequestreview-514843497