[Feature] Basic systemd service monitoring (#1153)

* basic systemd service monitoring

* update to work after /internal rename

* monitor systemd service cpu and memory usage

---------

Co-authored-by: henrygd <hank@henrygd.me>

agent/systemd_unsupported.go

@@ -0,0 +1,18 @@
+//go:build !linux
+
+package agent
+
+import "github.com/henrygd/beszel/internal/entities/systemd"
+
+// systemdManager manages the collection of systemd service statistics.
+type systemdManager struct{}
+
+// newSystemdManager creates a new systemdManager.
+func newSystemdManager() (*systemdManager, error) {
+	return &systemdManager{}, nil
+}
+
+// getServiceStats returns nil for non-linux systems.
+func (sm *systemdManager) getServiceStats() []*systemd.Service {
+	return nil
+}