Appender: support nested STRUCTs (1.4)

This is a backport of the PR duckdb#396 to `v1.4-andium` stable branch.

This PR adds support to nested `STRUCT` and `UNION` columns in
`Appender`. There are no nesting limit enforced on Java side.

Testing: existing test is extended to cover nested `STRUCT`s.

Author: staticlibs