SparkPost: error on features incompatible with template_id

Raise an `AnymailUnsupportedFeature` error
when trying to use a `template_id` along with
other content payload fields that SparkPost
silently ignores when template_id is present.

tests/test_sparkpost_integration.py

@@ -153,6 +153,7 @@ class SparkPostBackendIntegrationTests(AnymailTestMixin, SimpleTestCase):
                 "order": "12345",
             },
         )
+        message.from_email = None  # from_email must come from stored template
         message.send()
         recipient_status = message.anymail_status.recipients
         self.assertEqual(