nixos/journaldriver: add module for journaldriver log forwarder

Adds a module for running the journaldriver log forwarding agent via
systemd.

The agent can be deployed on both GCP instances and machines hosted
elsewhere to forward all logs from journald to Stackdriver Logging.

Consult the module options and upstream documentation for more
information.

Implementation notes:

* The service unit is configured to use systemd's dynamic user feature
  which will let systemd set up the state directory and appropriate
  user configuration at unit launch time instead of hardcoding it.

* The module depends on `network-online.target` to prevent a situation
  where journaldriver is failing and restarting multiple times before
  the network is online.