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-c.sym

@@ -170,6 +170,7 @@ JSONC_0.15 {
 JSONC_0.16 {
   global:
     json_object_array_insert_idx;
+    json_patch_apply;
 } JSONC_0.15;
 
 JSONC_0.17 {