SQL to_json should support all column types

The built-in SQL UDF "to_json" currently supports serializing StructType columns, as well as Arrays of StructType columns. If you attempt to use it on a different type, for example a map, you get an error like this:

AnalysisException: cannot resolve 'structstojson(`tags`)' due to data type mismatch: Input type map<string,string> must be a struct or array of structs.;;

This limitation seems arbitrary; if I were to go through the effort of enclosing my map in a struct, it would be serializable. Same thing with any other non-struct type.

Therefore the desired improvement is to allow to_json to operate directly on any column type. The associated code is here.