This diff handles setting up the endpoints and responder for actually fetching the media for a given thread. The changes made are:
1. Set up the endpoint (`keyserver/src/endpoints.js`) and the type (`lib/types/endpoints.js`)
2. Use `callServerEndpoint` in `lib/actions/thread-actions.js` to hit the newly created endpoint,
and route to the apprpriate responder.
3. Handle the validation of the request and call `fetchMediaForThread` from the responder in
`keyserver/src/responders/thread-responders.js`
Linear: https://linear.app/comm/issue/ENG-2874/set-up-the-endpoints-and-responder-for-retrieving-media
Depends on D6485