✨(domains) add support email field on MailDomain

Add new field on MailDomain to allow contact support
if some actions are required to fix domain.

src/backend/mailbox_manager/api/client/serializers.py

@@ -70,6 +70,7 @@ class MailDomainSerializer(serializers.ModelSerializer):
             "created_at",
             "updated_at",
             "count_mailboxes",
+            "support_email",
         ]
         read_only_fields = [
             "id",

src/backend/mailbox_manager/api/client/viewsets.py

@@ -33,6 +33,7 @@ class MailDomainViewSet(
 
     POST /api/<version>/mail-domains/ with expected data:
         - name: str
+        - support_email: str
         Return newly created domain
     """