func_json: Adds JSON_DECODE function

Adds the JSON_DECODE function for parsing JSON in the
dialplan. JSON parsing already exists in the Asterisk
core and is used for many different things. This
function exposes the basic parsing capability to
the user in the dialplan, for instance, in conjunction
with CURL for using API responses.

ASTERISK-29706 #close

Change-Id: Iea60c49a7358dfdc2db60803cdc9a742f808ba2c