[lib] Add logic for handling faonut actions

Summary:
issue: https://linear.app/comm/issue/ENG-4728/refactor-useservercall-step-3-implement-fanout-logic
Fanout actions are actions that call all keyservers.

Test Plan:
Created an object for searchMessages:

const searchMessagesActionObj: KeyserverCall<
  [SearchMessagesRequest],
  SearchMessagesResponse,
> = {
  actionFunc: searchMessages,
  config: { keyserverSelection: 'fanout' },
};

Called this action from the message search modal, checked that the results appear, and that it is possible to load more results

Reviewers: michal, kamil, ginsu, ashoat

Reviewed By: michal, ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9217