fix(json): Ensure input is parsed as json for json extract functions (facebookincubator#12409)

Summary:

We recently noticed as detailed in prestodb/presto#24563
that some json functions that take varchar as input and output a json type can skip
parsing the input. This can result in non-canonicalized result and inconsistent behavior.
This change ensures that json_extract and json_extract_scalar always parse the input
in these instances.

Differential Revision: D69611952

Author: Bikramjeet Vig