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_patch.h

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2021 Alexadru Ardelean.
+ *
+ * This is free software; you can redistribute it and/or modify
+ * it under the terms of the MIT license. See COPYING for details.
+ *
+ */
+
+/**
+ * @file
+ * @brief JSON Patch (RFC 6902) implementation for manipulating JSON objects
+ */
+#ifndef _json_patch_h_
+#define _json_patch_h_
+
+#include "json_pointer.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Apply the JSON patch to the base object.
+ * The patch object must be formatted as per RFC 6902.
+ * If the patch is not correctly formatted, an error will
+ * be returned.
+ *
+ * The original `base` object will first be copied, and then
+ * the patch will be applied.
+ * If anything fails during patching, the `res` object will be
+ * NULL and the function will return a negative result.
+ *
+ * @param base the JSON object which to patch
+ * @param patch the JSON object that describes the patch to be applied
+ * @param the resulting patched JSON object
+ *
+ * @return negative if an error (or not found), or 0 if succeeded
+ */
+JSON_EXPORT int json_patch_apply(struct json_object *base, struct json_object *patch,
+                                 struct json_object **res);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif