[lib][keyserver] Move upload output validators to lib
Summary:
upload_multimedia is listed in lib as an endpoint. We don't have an endpoint for it in the endpoint.js on the keyserver, but we have code that adds a handler
keyserverRouter.post( '/upload_multimedia', multerProcessor, uploadHandler(multimediaUploadResponder), );
I am moving this validator as well so that I can type something on lib corrreclty in later diffs - I will need to be able to list validators for all endpoints as specifiec on lib, so including upload_multimedia
Test Plan: flow, checked that mulimedia upload works
Reviewers: tomek, kamil, bartek
Reviewed By: tomek
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D12029