feat(bind9): support systemd drop-ins

Keep role configuration namespaced and allow hosts to customize systemd startup.

Co-authored-by: Copilot <copilot@github.com>

defaults/main.yaml

@@ -110,6 +110,10 @@ bind9_statics:
     acls:
     - "localhost"
 
+bind9_systemd_dropin: ~
+# [Service]
+# ExecStart=
+# ExecStart={{ bind9_binary }} -f -u {{ bind9_unix_user }} -4
 
 bind9_tsigkeys: []
 # - name: "name"