[SPARK-11329][SQL] Support star expansion for structs.

1. Supporting expanding structs in Projections. i.e.
  "SELECT s.*" where s is a struct type.
  This is fixed by allowing the expand function to handle structs in addition to tables.

2. Supporting expanding * inside aggregate functions of structs.
   "SELECT max(struct(col1, structCol.*))"
   This requires recursively expanding the expressions. In this case, it it the aggregate
   expression "max(...)" and we need to recursively expand its children inputs.

Author: Nong Li <nongli@gmail.com>

Closes #9343 from nongli/spark-11329.