[SQL] Add String option for DSL AS

Author: Michael Armbrust <michael@databricks.com>

Closes #3097 from marmbrus/asString and squashes the following commits:

6430520 [Michael Armbrust] Add String option for DSL AS

(cherry picked from commit 515abb9afa2d6b58947af6bb079a493b49d315ca)
Signed-off-by: Xiangrui Meng <meng@databricks.com>