feat: export initialMessagesFetch callback from hook

[AhyoungRyu]

Addresses https://sendbird.atlassian.net/browse/AC-690

I took the logic inside of useInitialMessagesFetch's effect hook and made a callback function. And made the function to be returned from the hook to make it be from outside of UIKit.

tl;dr

Before

function useInitialMessagesFetch(params): void {
  useEffect(() => {
     all the logics..
  }, [deps]}
}

After

function useInitialMessagesFetch(params): () => void {
  const fetchMessages() => useCallback(() => {
    all the logics..
  , [deps]}
  useEffect(() => {
     fetchMessages();
  }, [fetchMessages]}
  
  return fetchMessages;
}