Add broadcast channel to service schema

This will show which channel is configured, if any, for a service. It
mimics what we are doing for the `allowed_broadcast_provider`.

tests/app/service/test_rest.py

@@ -243,6 +243,7 @@ def test_get_service_by_id(admin_request, sample_service):
     assert json_resp['data']['email_branding'] is None
     assert json_resp['data']['prefix_sms'] is True
     assert json_resp['data']['allowed_broadcast_provider'] is None
+    assert json_resp['data']['broadcast_channel'] is None
 
     assert set(json_resp['data'].keys()) == {
         'active',
@@ -250,6 +251,7 @@ def test_get_service_by_id(admin_request, sample_service):
         'billing_contact_email_addresses',
         'billing_contact_names',
         'billing_reference',
+        'broadcast_channel',
         'consent_to_research',
         'contact_link',
         'count_as_live',