[keyserver/lib] Set up responders, endpoints, and related functions to fetch all pinned messages

Summary:
Following D7110, this diff handles introducing the necessary endpoint and responders to call the query.

Depends on D7110

Linear: https://linear.app/comm/issue/ENG-3189/set-up-responders-endpoints-and-related-functions-to-fetch-all-pinned

Test Plan:
Ran the following code to test, and confirmed output in my console (only including the relevant testing code).

import { fetchPinnedMessages } from 'lib/actions/message-actions.js';

const callFetchPinnedMessages = useServerCall(fetchPinnedMessages);

React.useEffect(() => {
  const getPins = async () => {
    const pins = await callFetchPinnedMessages({ threadID });
    console.log(pins);
  };
  getPins();
}, [...]);

Reviewers: atul, ginsu, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D7112