Include Full Fee Amount For Limit Orders

[nlordell]

Fixes #770

This PR makes sure limit orders also set their full_fee_amount. Specifically, CoW Protocol orders have 2 different fee fields:

• fee_amount: This is the actual fee amount that we charge the user. In order to make trading more attractive on CoW Swap, we have a few "knobs" for configuring subsidies on orders used to compute this amount. For limit orders, this is the analogous to surplus_fee which is the actual fee we change the user (but instead of it being a pro-rated fee, it is taken from the order's surplus).
• full_fee_amount: This is the actual expected "cost" of executing the order. We store this for a couple of reasons:
  1. Primary reason is that we want to use this value in the objective function and not the actual fee amount. The reason for this is we want competition to be done without taking subsidies into account.
  2. Secondary reason (and much less important) is to help in debugging and make sure our fee estimation is working as expected and somewhat accurate (we should compare these values with the actual on-chain gas costs of settlement executions).

These fields should be accurate for limit orders now 🎉.

Test Plan

Unit test to check that this field is being set. It gets read in the same way that it gets read for regular orders during auction building.

[sistemd]

Types are awesome. The fact that this is in the sell token should be reflected by the type IMO. That'd be really nice. But obviously outside the scope of this PR.

[nlordell]

The fact that this is in the sell token should be reflected by the type IMO.

That would be the holy grail! We used to have a graph-based price fining algorithm for the first version of this project (Gnosis Protocol it was called) - and mixing up amounts for different tokens was one of the primary causes of logic bugs.