MSC3873: Escape keys when flattening dicts. (#15004)

This disambiguates keys which attempt to match fields
with a dot in them (e.g. m.relates_to).

Disabled by default behind an experimental configuration flag.

tests/push/test_push_rule_evaluator.py

@@ -48,6 +48,14 @@ class FlattenDictTestCase(unittest.TestCase):
         input = {"foo": {"bar": "abc"}}
         self.assertEqual({"foo.bar": "abc"}, _flatten_dict(input))
 
+        # If a field has a dot in it, escape it.
+        input = {"m.foo": {"b\\ar": "abc"}}
+        self.assertEqual({"m.foo.b\\ar": "abc"}, _flatten_dict(input))
+        self.assertEqual(
+            {"m\\.foo.b\\\\ar": "abc"},
+            _flatten_dict(input, msc3783_escape_event_match_key=True),
+        )
+
     def test_non_string(self) -> None:
         """Non-string items are dropped."""
         input: Dict[str, Any] = {