This function extends JSON support with fromJson() function,
which can be used in UPDATE clause to transform JSON value
into a value with proper CQL type.
fromJson() accepts strings and may return any type, so its instances,
like toJson(), are generated during calls.
This commit also extends functions::get() with additional
'receiver' parameter. This parameter is used to extract receiver type
information neeeded to generate proper fromJson instance.
Receiver is known only during insert/update, so functions::get() also
accepts a nullptr if receiver is not known (e.g. during selection).
References #2058
cd76a01747