Add support for configuring Synapse's MSC4140 (Delayed Events) experimental feature

This is required by Element Call.

Ref:

- 93ae2aed98/docs/self-hosting.md (a-matrix-homeserver)
- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/3562

roles/custom/matrix-synapse/defaults/main.yml

@@ -1261,6 +1261,20 @@ matrix_synapse_experimental_features_msc4108_enabled: false
 # Set this to false if you still want to disable to API for some reason. Note that doing so breaks Element X compatibility though.
 matrix_synapse_experimental_features_msc3266_enabled: true
 
+# Controls whether to enable the "Delayed Events" experimental feature.
+# Delayed events are required for proper call (Element Call) participation signalling.
+# If disabled it is very likely that you end up with stuck calls in Matrix rooms.
+#
+# Related to `matrix_synapse_max_event_delay_duration`
+#
+# See https://github.com/matrix-org/matrix-spec-proposals/pull/4140
+matrix_synapse_experimental_features_msc4140_enabled: false
+
+# Controls the maximum allowed duration by which sent events can be delayed, as per MSC4140.
+#
+# See `matrix_synapse_experimental_features_msc4140_enabled`.
+matrix_synapse_max_event_delay_duration: 24h
+
 # Enable this to activate the REST auth password provider module.
 # See: https://github.com/ma1uta/matrix-synapse-rest-password-provider
 matrix_synapse_ext_password_provider_rest_auth_enabled: false

roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -2987,5 +2987,12 @@ experimental_features:
   {% if matrix_synapse_experimental_features_msc4108_enabled %}
   msc4108_enabled: true
   {% endif %}
+  {% if matrix_synapse_experimental_features_msc4140_enabled %}
+  msc4140_enabled: true
+  {% endif %}
+
+{% if matrix_synapse_experimental_features_msc4140_enabled %}
+max_event_delay_duration: {{ matrix_synapse_max_event_delay_duration | to_json }}
+{% endif %}
 
 # vim:ft=yaml