Support MSC4140: Delayed events

Also implement "parent" delayed events, which were in a previous version
of the MSC and may be reintroduced or be part of a new MSC later.

changelog.d/17326.feature

@@ -0,0 +1,2 @@
+Add initial implementation of delayed events as proposed by [MSC4140](https://github.com/matrix-org/matrix-spec-proposals/pull/4140).
+

synapse/config/experimental.py

@@ -2,7 +2,7 @@
 # This file is licensed under the Affero General Public License (AGPL) version 3.
 #
 # Copyright 2021 The Matrix.org Foundation C.I.C.
-# Copyright (C) 2023 New Vector, Ltd
+# Copyright (C) 2023-2024 New Vector, Ltd
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU Affero General Public License as
@@ -443,6 +443,20 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
             "msc3823_account_suspension", False
         )
 
+        # MSC4140: Delayed events
+        # The maximum allowed duration for delayed events.
+        try:
+            self.msc4140_max_delay = int(experimental["msc4140_max_delay"])
+            if self.msc4140_max_delay <= 0:
+                raise ValueError
+        except ValueError:
+            raise ConfigError(
+                "msc4140_max_delay must be a positive integer",
+                ("experimental", "msc4140_max_delay"),
+            )
+        except KeyError:
+            self.msc4140_max_delay = 10 * 365 * 24 * 60 * 60 * 1000  # 10 years
+
         # MSC4151: Report room API (Client-Server API)
         self.msc4151_enabled: bool = experimental.get("msc4151_enabled", False)