Send message format to CBC

Either cap or ibag

app/models.py

@@ -2453,6 +2453,11 @@ class BroadcastProvider:
     PROVIDERS = [EE, VODAFONE, THREE, O2]
 
 
+class BroadcastProviderMessageType:
+    CBC = 'cbc'
+    IBAG = 'ibag'
+
+
 class BroadcastProviderMessageStatus:
     TECHNICAL_FAILURE = 'technical-failure'  # Couldn’t send (cbc proxy 5xx/4xx)
     SENDING = 'sending'  # Sent to cbc, awaiting response