✨(mailboxes) add mail provisioning api integration

We want people to create new mailboxes in La Régie.
This commit adds integration with intermediary dimail-api,
which will in turn send our email creation request to Open-Xchange.

src/backend/mailbox_manager/api/serializers.py

@@ -11,6 +11,8 @@ class MailboxSerializer(serializers.ModelSerializer):
     class Meta:
         model = models.Mailbox
         fields = ["id", "first_name", "last_name", "local_part", "secondary_email"]
+        # everything is actually read-only as we do not allow update for now
+        read_only_fields = ["id"]
 
 
 class MailDomainSerializer(serializers.ModelSerializer):

src/backend/mailbox_manager/api/viewsets.py

@@ -74,7 +74,18 @@ class MailBoxViewSet(
     mixins.ListModelMixin,
     viewsets.GenericViewSet,
 ):
-    """MailBox ViewSet"""
+    """MailBox ViewSet
+
+    GET /api/<version>/mail-domains/<domain-slug>/mailboxes/
+        Return a list of mailboxes on the domain
+
+    POST /api/<version>/mail-domains/<domain-slug>/mailboxes/ with expected data:
+        - first_name: str
+        - last_name: str
+        - local_part: str
+        - secondary_email: str
+        Sends request to email provisioning API and returns newly created mailbox
+    """
 
     permission_classes = [permissions.MailBoxPermission]
     serializer_class = serializers.MailboxSerializer