[wsrp] [change request #253] Drop many extensions fields

Document: Spec v0.92

Sections: throughout the data structures

Requested By: Alejandro Abdelnur

Old text: all data structures have extension fields

New text: drop the extension fields from all but the
top level structures (the ones that are directly used as parameters of
the method calls).

Reasoning: Simplification. IMO it's overkilling having
extensions in all data structures.