Add bytea_agg, parallel to string_agg.

doc/src/sgml/func.sgml

src/backend/utils/adt/varlena.c

src/include/catalog/catversion.h

src/include/catalog/pg_aggregate.h

src/include/catalog/pg_proc.h

src/include/utils/builtins.h

src/test/regress/expected/aggregates.out

src/test/regress/sql/aggregates.sql