[FEATURE] Add TO_JSON_STRING and ARRAY_LENGHT functions and refactor the JSON function

[LantaoJin]

Is your feature request related to a problem?
A followup of #667

1. Refactor JSON: Currently the JSON evaluates STRING/JSON_ARRAY/JSON_OBJECT. It brings many confusions. In Splunk, the data could be stored as JSON object, JSON function evaluates JSON object. But in Spark, there is no JSON object, but it contains STRING, StructType and ArrayType. So the JSON function can only evaluate STRING types.

2. Add ARRAY_LENGHT: for the same reason above, we separate JSON_ARRAY_LENGHT to JSON_ARRAY_LENGHT and ARRAY_LENGHT. JSON_ARRAY_LENGHT only accepts STRING type, ARRAY_LENGHT only accepts ArrayType.

3. Add TO_JSON_STRING: After the refactor of (1), we still need a method to convert JSON_ARRAY/JSON_OBJECT to valid JSON STRING. TO_JSON_STRING accepts both StructType and ArrayType as input and returns JSON formatted string.