feat: add SMTP submission listener and enhance stalwart configuration

- add SMTP submission listener on port 587 with TLS requirement
- configure HAProxy frontend/backend for submission with send-proxy-v2 support
- add send-proxy-v2 support to all listeners
- add dynamic HAProxy node discovery for proxy trusted networks
- use service hostname instead of node FQDN for autoconfig/autodiscover
- remove redundant IMAP/IMAPS/SMTP alt-names from TLS certificates
- update VRRP CNAME configuration to use mail.main.unkin.net

hieradata/roles/infra/halb/haproxy2.yaml

@@ -196,6 +196,17 @@ profiles::haproxy::frontends:
       tcp-request:
         - inspect-delay 5s
         - content accept if { req_len 0 }
+  fe_submission:
+    description: 'Frontend for Stalwart SMTP Submission'
+    bind:
+      0.0.0.0:587: []
+    mode: 'tcp'
+    options:
+      log: global
+      default_backend: be_stalwart_submission
+      tcp-request:
+        - inspect-delay 5s
+        - content accept if { req_len 0 }
 
 profiles::haproxy::backends:
   be_letsencrypt: