Put parameters on the payload rather than under a parameters object

The other three SDKs expose a payload's parameters as fields alongside the
common ones, and so do this SDK's own generated single-entry methods
(`PostUserFundsAccount({ ik, ledgerIk, amount })`). Nesting them made one entry
type read two different ways depending on which method you reached for.

Nesting had avoided the collision a flat namespace allows, so handle it the way
§2.5 describes instead: the first occupant of a name keeps it, a later parameter
is suffixed, and it still posts under the name the Schema knows it by. Unlike
Python's, TypeScript's failure mode here is loud -- a duplicate property in a
generated type is a compile error -- so the escape only has to be correct, not
defensive.

Author: vigneshwerv