Add support for JSON column type

Vitess 9.0.0 added better VReplication support to the JSON column type. VStream will send the JSON string as is to the Vitess Connector. For example, if we insert the following as the value of the JSON column:

{"key1": "value1", "key2": {"key21": "value21", "key22": "value22"}}

VStream would send us exactly the same normalized JSON String now: 

{"key1":"value1","key2":{"key21":"value21","key22":"value22"}}

The support is seamless and no change to the Vitess Connector's codebase is needed. We need to add test to cover JSON column type and update the documentation.