Add support for configuring Synapse's MSC4222 (adding state_after to sync v2) experimental feature

This is required by Element Call.

Ref:

- https://github.com/element-hq/element-call/blob/93ae2aed9841e0b066d515c56bd4c122d2b591b2/docs/self-hosting.md#a-matrix-homeserver
- #3562

Author: spantaleev

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

@@ -1275,6 +1275,14 @@ matrix_synapse_experimental_features_msc4140_enabled: false
 # See `matrix_synapse_experimental_features_msc4140_enabled`.
 matrix_synapse_max_event_delay_duration: 24h
 
+# Controls whether to enable the MSC4222 experimental feature (adding `state_after` to sync v2).
+#
+# Allow clients to opt-in to a change of the sync v2 API that allows them to correctly track the state of the room.
+# This is required by Element Call to track room state reliably.
+#
+# See https://github.com/matrix-org/matrix-spec-proposals/pull/4222
+matrix_synapse_experimental_features_msc4222_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

@@ -2990,6 +2990,9 @@ experimental_features:
   {% if matrix_synapse_experimental_features_msc4140_enabled %}
   msc4140_enabled: true
   {% endif %}
+  {% if matrix_synapse_experimental_features_msc4222_enabled %}
+  msc4222_enabled: true
+  {% endif %}
 
 {% if matrix_synapse_experimental_features_msc4140_enabled %}
 max_event_delay_duration: {{ matrix_synapse_max_event_delay_duration | to_json }}