Draft: return redacted PDUs for inaccessible events over federation

https://gitlab.com/famedly/conduit/-/issues/370#note_1929732765

..., it seems like we should be sending redacted PDUs when a remote server asks for events it shouldn't have access to. There is some more behavior which I am not too certain about, and will be writing a spec pr or MSC to clarify the behavior fully.

https://spec.matrix.org/unstable/server-server-api/#get_matrixfederationv1backfillroomid

Event retrieval endpoints are not supposed to return errors which Grapevine currently does.