json_patch: add first implementation only with patch application

Initially I wanted to also do a function that generates the JSON patch from
two JSON documents, but even just applying the JSON patch was a bit of
work, especially when needing to satisfy all the test-cases.

This change defines all the operation in the RFC6902. The addition isn't
too big (for the json_patch_apply() function), as part of the heavy lifting
is also done by JSON pointer logic.

All the ops were tested with the test-cases defined at:
  https://github.com/json-patch/json-patch-tests

RFC6902: https://tools.ietf.org/html/rfc6902

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

json.h.cmakein

@@ -26,6 +26,7 @@ extern "C" {
 #include "json_c_version.h"
 #include "json_object.h"
 #include "json_object_iterator.h"
+@JSON_H_JSON_PATCH@
 @JSON_H_JSON_POINTER@
 #include "json_tokener.h"
 #include "json_util.h"