[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