A basic json_tuple implementation to address issue #1377 that is NOT a proper table generating
function as in Hive. Example:

presto:default> select json_tuple('{ "x": 1, "y": 2, "z": { "x":
"another_inner_x" }, "t": [1,2,3] }', '["x", "t"]');
        _col0
---------------------
 {"x":1,"t":[1,2,3]}

The second string argument specifies which fields to extract from the given
json string (this is a string as presto currently doesn't support
variable length arguments in functions).