Include whether the requesting user has participated in a thread. (#11577)

Per updates to MSC3440.

This is implement as a separate method since it needs to be cached
on a per-user basis, instead of a per-thread basis.

tests/rest/client/test_relations.py

@@ -515,6 +515,9 @@ class RelationsTestCase(unittest.HomeserverTestCase):
                 2,
                 actual[RelationTypes.THREAD].get("count"),
             )
+            self.assertTrue(
+                actual[RelationTypes.THREAD].get("current_user_participated")
+            )
             # The latest thread event has some fields that don't matter.
             self.assert_dict(
                 {