Stabilize set_displayname() in module API (#14628)

changelog.d/14629.feature

@@ -0,0 +1 @@
+Adds a stable `set_displayname()` method to the module API for setting a user's display name.

synapse/module_api/__init__.py

@@ -1547,6 +1547,37 @@ async def create_room(
 
         return room_id_and_alias["room_id"], room_id_and_alias.get("room_alias", None)
 
+    async def set_displayname(
+        self,
+        target_user: UserID,
+        requester_user_id: UserID,
+        new_displayname: str,
+        deactivation: bool = False,
+    ) -> None:
+        """Sets a user's display name.
+
+        Added in Synapse v1.74.0.
+
+        Args:
+            target_user:
+                The user whose display name is to be changed.
+            requester:
+                The user attempting to make this change.
+            new_displayname:
+                The new display name to give the user.
+            deactivation:
+                Whether this change was made while deactivating the user.
+        """
+        requester = create_requester(requester_user_id)
+        by_admin = await self.is_user_admin(requester_user_id.to_string())
+        await self._hs.get_profile_handler().set_displayname(
+            target_user=target_user,
+            requester=requester,
+            new_displayname=new_displayname,
+            by_admin=by_admin,
+            deactivation=deactivation,
+        )
+
 
 class PublicRoomListManager:
     """Contains methods for adding to, removing from and querying whether a room