Add new module API for adding custom fields to events unsigned section (#16549)

docs/modules/add_extra_fields_to_client_events_unsigned.md

@@ -0,0 +1,32 @@
+# Add extra fields to client events unsigned section callbacks
+
+_First introduced in Synapse v1.96.0_
+
+This callback allows modules to add extra fields to the unsigned section of
+events when they get sent down to clients.
+
+These get called *every* time an event is to be sent to clients, so care should
+be taken to ensure with respect to performance.
+
+### API
+
+To register the callback, use
+`register_add_extra_fields_to_unsigned_client_event_callbacks` on the
+`ModuleApi`.
+
+The callback should be of the form
+
+```python
+async def add_field_to_unsigned(
+    event: EventBase,
+) -> JsonDict:
+```
+
+where the extra fields to add to the event's unsigned section is returned.
+(Modules must not attempt to modify the `event` directly).
+
+This cannot be used to alter the "core" fields in the unsigned section emitted
+by Synapse itself.
+
+If multiple such callbacks try to add the same field to an event's unsigned
+section, the last-registered callback wins.