netutils/cJSON: add input stream JSON parser

Patch provides new stream parsing engine. Benefits are: parsing input
string using user supplied getc function, which allows reading input
directly from files or socket streams (thus avoid temporary buffer
allocation). Parser has also been fixed to pass all tests in 'json_gtest'
folder.

From github:thingsee/thingsee-sdk pull request:

 "Improve cJSON to allow parse/print from/to stream (caller supplied getc/putc functions) for reducing peak memory usage"

 https://github.com/thingsee/thingsee-sdk/pull/6



Signed-off-by: Jussi Kivilinna <jussi.kivilinna@haltian.com>
(cherry picked from commit ed7c67771fcc97ffc9726241d6db70b24b0c0995)