feat: grant predefined roles to postgres by default

`postgres` can already grant these roles to itself; granting these by
default removes one step if users need to read Storage migrations, etc.

Author: soedirgo

migrations/db/migrations/20240606060239_grant_predefined_roles_to_postgres.sql

@@ -0,0 +1,4 @@
+-- migrate:up
+grant pg_read_all_data, pg_signal_backend to postgres;
+
+-- migrate:down