Add normalization_context option for Mercure (#1189)

Author: alanpoulain

core/mercure.md

@@ -124,3 +124,4 @@ In addition to `private`, the following options are available:
 * `id`: the SSE id of this event, if not set the ID will be generated by the mercure Hub
 * `type`: the SSE type of this event, if not set this field is omitted
 * `retry`: the `retry` field of the SSE, if not set this field is omitted
+* `normalization_context`: the specific normalization context to use for the update.