Simplify API for JSON access in DataFrames/SQL

get_json_object() is a UDF to parse JSON fields. It is verbose and hard to use, e.g. I wasn't expecting the path to a field to have to start with "$.".

We can simplify all of this when a column is of StringType, and a nested field is requested. This API sugar will in the query planner be rewritten as get_json_object.

This nested access can then be extended in the future to other semi-structured formats.