Extract schema information from the underlying connections, add Arrow schema conversion utility

[geoffxy]

This PR modifies BRAD's connection code to extract schema information (the column names and types) from the underlying database connections. Each database has slightly different types, so we unify them in a "best effort" way for now. Later on we will have better support for specific SQL dialects.

High level summary of the changes

• Add result_schema() to BRAD cursors
• Define BRAD-specific DataType, Field and Schema classes
• Add a C++ helper function that converts a Python BRAD Schema into an Arrow schema
• Various minor modifications to pass the Python schema to the Flight SQL code

[geoffxy]

@sopzha This converts the Python BRAD schema into an Arrow schema that you can use. The result_schema variable is currently unused, but you can incorporate it into the rest of your code.