REQS-334 moved trade wrapper to platform and created specific fudge buil...

...der
As discussed, I don't think this actually needs to implement Trade as long as we have access to the underlying trade via the wrapper. This then avoids the complications around the FudgeBuilder (that change is no longer needed) and the code can be moved back to the Sesame repo. 

What is still required is that we're stating that implementing classes are immutable but the underlying trade is mutable (in this version by the set/add Attributes methods you added, in the proposed version by the same methods directly on the trade). There are 2 options for this:
 - we create an immutable wrapper for the trades that is automatically used inside the TradeWrappers which throws exceptions if mutation is attempted (i.e. in a similar way to Guava's ImmutableList)
 - we always return a defensive copy of the trade such that mutating it has no effect on the held object

As it only applies to the attributes it is probably a case of doing whichever is simpler