Add media repository callbacks to module API to control media upload size (#18457)

Adds new callbacks for media related functionality:

- `get_media_config_for_user`
- `is_user_allowed_to_upload_media_of_size`

docs/modules/media_repository_callbacks.md

@@ -0,0 +1,56 @@
+# Media repository callbacks
+
+Media repository callbacks allow module developers to customise the behaviour of the
+media repository on a per user basis. Media repository callbacks can be registered
+using the module API's `register_media_repository_callbacks` method.
+
+The available media repository callbacks are:
+
+### `get_media_config_for_user`
+
+_First introduced in Synapse v1.132.0_
+
+```python
+async def get_media_config_for_user(user_id: str) -> Optional[JsonDict]
+```
+
+Called when processing a request from a client for the
+[media config endpoint](https://spec.matrix.org/latest/client-server-api/#get_matrixclientv1mediaconfig).
+
+The arguments passed to this callback are:
+
+* `user_id`: The Matrix user ID of the user (e.g. `@alice:example.com`) making the request.
+
+If the callback returns a dictionary then it will be used as the body of the response to the
+client.
+
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns `None`, Synapse falls through to the next one. The value of the first
+callback that does not return `None` will be used. If this happens, Synapse will not call
+any of the subsequent implementations of this callback.
+
+If no module returns a non-`None` value then the default media config will be returned.
+
+### `is_user_allowed_to_upload_media_of_size`
+
+_First introduced in Synapse v1.132.0_
+
+```python
+async def is_user_allowed_to_upload_media_of_size(user_id: str, size: int) -> bool
+```
+
+Called before media is accepted for upload from a user, in case the module needs to
+enforce a different limit for the particular user.
+
+The arguments passed to this callback are:
+
+* `user_id`: The Matrix user ID of the user (e.g. `@alice:example.com`) making the request.
+* `size`: The size in bytes of media that is being requested to upload.
+
+If the module returns `False`, the current request will be denied with the error code
+`M_TOO_LARGE` and the HTTP status code 413.
+
+If multiple modules implement this callback, they will be considered in order. If a callback
+returns `True`, Synapse falls through to the next one. The value of the first callback that
+returns `False` will be used. If this happens, Synapse will not call any of the subsequent
+implementations of this callback.