services: postgresql: Add postgresql-role-service-type.

* gnu/services/databases.scm (postgresql-role,
postgresql-role?, postgresql-role-name,
postgresql-role-permissions, postgresql-role-create-database?,
postgresql-role-configuration, postgresql-role-configuration?,
postgresql-role-configuration-host, postgresql-role-configuration-roles,
postgresql-role-service-type): New procedures.
* gnu/tests/databases.scm: Test it.
* doc/guix.texi: Document it.