* postgresql: reorganize package and it's extensions
Extracts some useful parts of https://github.com/NixOS/nixpkgs/pull/38698,
in particular, it's vision that postgresql plugins should be namespaced.
For large setups it is useful to list all databases explicit
(for example if temporary databases are also present) and store them in extra
files.
For smaller setups it is more convenient to just backup all databases at once,
because it is easy to forget to update configuration when adding/renaming
databases. pg_dumpall also has the advantage that it backups users/passwords.
As a result the module becomes easier to use because it is sufficient
in the default case to just set one option (services.postgresqlBackup.enable).
1. Needs to call makeTest or else nothing happens when you run
`nix-build nixos/tests/postgresql.nix`.
2. Tests run as root, so there needs to be a corresponding user in
PostgreSQL.
