Update serialization and service schema

- added id and service_id in serialization
- added 'letter_contacts' to the exluded list for marshmallow service schema

app/schemas.py

@@ -209,6 +209,7 @@ class ServiceSchema(BaseSchema):
             'service_sms_senders',
             'monthly_billing',
             'reply_to_email_addresses',
+            'letter_contacts',
         )
         strict = True