handle "standalone" databases (or single-database-instance)

Single-database-instances is a quite common use case nowadays (some call this a "micro-service instance") so we should expose some commands and APIs to manage such situations.

• At database creation, when no instance is specified, create an eponymous instance.
• At database update, assuming the underlying instance is named after the database.
• For satellite services, e.g. pgbackrest, name them according to the database name.
• Have dedicated role created after the database name, with only permission on that database.
• See how instances-related options would be meaningful or not in this configuration, esp. "roles" (should these be roles only permitted on the database at stake?)
• (probably more things to do, to be determined during the implementation)