Add a new admin API to create a new device for a user. (#15611)

This allows an external service (e.g. the matrix-authentication-service)
to create devices for users.

changelog.d/15611.feature

@@ -0,0 +1 @@
+Add a new admin API to create a new device for a user.

docs/admin_api/user_admin_api.md

@@ -813,6 +813,33 @@ The following fields are returned in the JSON response body:
 
 - `total` - Total number of user's devices.
 
+### Create a device
+
+Creates a new device for a specific `user_id` and `device_id`. Does nothing if the `device_id` 
+exists already.
+
+The API is:
+
+```
+POST /_synapse/admin/v2/users/<user_id>/devices
+
+{
+  "device_id": "QBUAZIFURK"
+}
+```
+
+An empty JSON dict is returned.
+
+**Parameters**
+
+The following parameters should be set in the URL:
+
+- `user_id` - fully qualified: for example, `@user:server.com`.
+
+The following fields are required in the JSON request body:
+
+- `device_id` - The device ID to create.
+
 ### Delete multiple devices
 Deletes the given devices for a specific `user_id`, and invalidates
 any access token associated with them.

synapse/rest/admin/devices.py

@@ -137,6 +137,35 @@ async def on_GET(
         devices = await self.device_handler.get_devices_by_user(target_user.to_string())
         return HTTPStatus.OK, {"devices": devices, "total": len(devices)}
 
+    async def on_POST(
+        self, request: SynapseRequest, user_id: str
+    ) -> Tuple[int, JsonDict]:
+        """Creates a new device for the user."""
+        await assert_requester_is_admin(self.auth, request)
+
+        target_user = UserID.from_string(user_id)
+        if not self.is_mine(target_user):
+            raise SynapseError(
+                HTTPStatus.BAD_REQUEST, "Can only create devices for local users"
+            )
+
+        u = await self.store.get_user_by_id(target_user.to_string())
+        if u is None:
+            raise NotFoundError("Unknown user")
+
+        body = parse_json_object_from_request(request)
+        device_id = body.get("device_id")
+        if not device_id:
+            raise SynapseError(HTTPStatus.BAD_REQUEST, "Missing device_id")
+        if not isinstance(device_id, str):
+            raise SynapseError(HTTPStatus.BAD_REQUEST, "device_id must be a string")
+
+        await self.device_handler.check_device_registered(
+            user_id=user_id, device_id=device_id
+        )
+
+        return HTTPStatus.CREATED, {}
+
 
 class DeleteDevicesRestServlet(RestServlet):
     """