DRILL-6676: Add Union, List and Repeated List types to Result Set Loader
Adds required functionalty to the list and repeated list vectors.
Row set accessor changes
Adds a "variant" type to model both unions and (non-repeated) lists (which can act as a repeated union, among other things.)
Adds union, list and repeated list support to the result set loader and associated classes.
Copied much of the general documentation from my private Wiki into mark-down files.
closes #1429
