Allow different strategies for duplicate keys in JSON objects

[YintongMa]

Both MySQL JSON parser and Rapidjson uses last-wins strategy for duplicate keys now. (see https://bugs.mysql.com/bug.php?id=86866). Could jsoncons provide an alternative option for different strategies? (last-wins/first-wins)
For example:
mysql> INSERT INTO t1 VALUES ('{"x": 17, "x": "red", "x": [3, 5, 7]}');

mysql> SELECT c1 FROM t1;
+------------------+
| c1 |
+------------------+
| {"x": [3, 5, 7]} |
+------------------+

Rapidjson has the same result. However it's '{"x": 17}' in jsoncons.