Add a utility function to extract variadic function arguments

This is epecially useful in the case or "VARIADIC ANY" functions. The
caller can get the artguments and types regardless of whether or not and
explicit VARIADIC array argument has been used. The function also
provides an option to convert arguments on type "unknown" to to "text".

Michael Paquier and me, reviewed by Tom Lane.

Backpatch to 9.4 in order to support the following json bug fix.