Add INSERT(..., DEFAULT, ).

src/backend/nodes/copyfuncs.c

src/backend/nodes/equalfuncs.c

src/backend/parser/analyze.c

src/backend/parser/gram.y

src/backend/parser/parse_target.c

src/include/nodes/nodes.h

src/include/nodes/parsenodes.h

src/test/regress/expected/insert.out

src/test/regress/parallel_schedule

src/test/regress/serial_schedule

src/test/regress/sql/insert.sql