Experimental support for MSC3772 (#12740)

Implements the following behind an experimental configuration flag:

* A new push rule kind for mutually related events.
* A new default push rule (`.m.rule.thread_reply`) under an unstable prefix.

This is missing part of MSC3772:

* The `.m.rule.thread_reply_to_me` push rule, this depends on MSC3664 / #11804.

synapse/storage/databases/main/push_rule.py

@@ -61,6 +61,11 @@ def _is_experimental_rule_enabled(
         and not experimental_config.msc3786_enabled
     ):
         return False
+    if (
+        rule_id == "global/underride/.org.matrix.msc3772.thread_reply"
+        and not experimental_config.msc3772_enabled
+    ):
+        return False
     return True