Add support for sticky events (experimental)

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

@@ -1383,6 +1383,13 @@ matrix_synapse_max_event_delay_duration: 24h
 # See https://github.com/matrix-org/matrix-spec-proposals/pull/4222
 matrix_synapse_experimental_features_msc4222_enabled: false
 
+# Controls whether to enable the MSC4354 experimental feature (sticky events).
+#
+# This is implemented since Synapse v1.148.0 and can be used by element-call v0.17.0+
+#
+# See https://github.com/matrix-org/matrix-spec-proposals/pull/4354
+matrix_synapse_experimental_features_msc4354_enabled: false
+
 # 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

@@ -3006,6 +3006,9 @@ experimental_features:
   {% if matrix_synapse_experimental_features_msc4222_enabled %}
   msc4222_enabled: true
   {% endif %}
+  {% if matrix_synapse_experimental_features_msc4354_enabled %}
+  msc4354_enabled: true
+  {% endif %}
 
 {% if matrix_synapse_experimental_features_msc4140_enabled %}
 max_event_delay_duration: {{ matrix_synapse_max_event_delay_duration | to_json }}