Update serialization and service schema

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

app/models.py

@@ -1406,6 +1406,8 @@ class ServiceLetterContact(db.Model):
 
     def serialize(self):
         return {
+            'id': str(self.id),
+            'service_id': str(self.service_id),
             'contact_block': self.contact_block,
             'is_default': self.is_default,
             'created_at': self.created_at.strftime(DATETIME_FORMAT),