CBE: Move the definition of the external structures into the exporter classes

See MDL-52064.

Sorry about the brief description - this will be some internal refactoring so we can auto-generate much of the external function descriptions/responses automatically. This is important because:
a) Remove code duplication
b) Remove mistakes
c) Remove inconsistencies

The end result will that any persistent object will have it's structure generated automatically - so it will have the same signature in every external function it is listed as a parameter or response.