DRILL-7377: Nested schemas for dynamic EVF columns
The Result Set Loader (part of EVF) allows adding columns up-front
before reading rows (so-called "early schema.") Such schemas allow
nested columns (maps with members, repeated lists with a type, etc.)
The Result Set Loader also allows adding columns dynamically
while loading data (so-called "late schema".) Previously, the code
assumed that columns would be added top-down: first the map, then
the map's contents, etc.
Charles found a need to allow adding a nested column (a repeated
list with a declared list type.)
This patch revises the code to use the same mechanism in both the
early- and late-schema cases, allowing adding nested columns at
any time.
Testing: Added a new unit test case for the repeated list late
schema with content case.
