Uniformize spam-checker API, part 2: check_event_for_spam (#12808)

Signed-off-by: David Teller <davidt@element.io>

synapse/module_api/__init__.py

@@ -35,6 +35,7 @@ from typing_extensions import ParamSpec
 from twisted.internet import defer
 from twisted.web.resource import Resource
 
+from synapse import spam_checker_api
 from synapse.api.errors import SynapseError
 from synapse.events import EventBase
 from synapse.events.presence_router import (
@@ -140,6 +141,9 @@ are loaded into Synapse.
 
 PRESENCE_ALL_USERS = PresenceRouter.ALL_USERS
 
+ALLOW = spam_checker_api.Allow.ALLOW
+# Singleton value used to mark a message as permitted.
+
 __all__ = [
     "errors",
     "make_deferred_yieldable",
@@ -147,6 +151,7 @@ __all__ = [
     "respond_with_html",
     "run_in_background",
     "cached",
+    "Allow",
     "UserID",
     "DatabasePool",
     "LoggingTransaction",