Adds an `extraServiceConfig` option to the NixOS module that is merged into the systemd service's `serviceConfig` after the module's defaults. This lets operators set arbitrary `[Service]` settings, most notably resource limits like `MemoryMax` and `CPUQuota`, without needing to fork the module, and also lets them override any of the defaults we set out of the box (e.g. to relax a sandboxing knob). Implemented as `attrsOf unspecified` merged with `//` so the user's attrs win on conflict.