Add SET SESSION AUTHORIZATION command.

doc/src/sgml/ref/allfiles.sgml

doc/src/sgml/ref/set_session_auth.sgml

doc/src/sgml/reference.sgml

src/backend/commands/variable.c

src/backend/parser/gram.y

src/backend/parser/keywords.c

src/backend/utils/init/miscinit.c

src/backend/utils/init/postinit.c

src/bin/psql/tab-complete.c

src/include/miscadmin.h