Add V2 inbound_sms API

- had to update the serialization in the model so that the date time is appended with the UTC timezone
- test has been added to ensure that the schema will validate the response correctly

tests/app/conftest.py

@@ -47,6 +47,7 @@ from tests.app.db import (
     create_api_key,
     create_inbound_number,
     create_letter_contact,
+    create_inbound_sms,
 )
 
 
@@ -1014,6 +1015,11 @@ def sample_inbound_numbers(notify_db, notify_db_session, sample_service):
     return inbound_numbers
 
 
+@pytest.fixture
+def sample_inbound_sms(notify_db, notify_db_session, sample_service):
+    return create_inbound_sms(sample_service)
+
+
 @pytest.fixture
 def restore_provider_details(notify_db, notify_db_session):
     """