WIP, but main extractor paths seem to work

This adds a possible macro for building JDBC type sources.

Right now just a size annotation, also might need an unsigned one.

Number of types available probably needs to increase. But it seems to build most of the info we would need for a source
I think String/Int types will have to expand more, so an either probably doesn't quite do it. We can use another sentinel type though to put here that inherits from  JdbcColumn. Something like JdbcValueType extends JdbcColumn and have OptionalColumn accept it?