Add automatic handling of RATE_LIMIT_HIT errors

[tanmayg06]

This PR adds automatic handling of RATE_LIMIT_HIT errors in the Zulip API client. When a rate limit error is encountered, the client will now automatically wait for the specified period (from the Retry-After header if available) and retry the request, rather than requiring each application to implement this logic separately.

Fixes: The issue where applications need to implement their own rate limit handling logic when using the Zulip API client.

Changes include:

1. Enhanced do_api_query method to detect RATE_LIMIT_HIT errors
2. Added logic to extract and use the Retry-After header value
3. Implemented automatic retry mechanism with appropriate sleep duration
4. Added fallback to existing error retry mechanism when no Retry-After header is provided
5. Added comprehensive tests for the new functionality
6. Updated README.md with documentation about the automatic rate limit handling

Code example of the implementation:

# Handle rate limiting automatically
if json_result.get("result") == "error" and json_result.get("code") == "RATE_LIMIT_HIT":
    retry_after = None
    # Check for Retry-After header (in seconds)
    if "Retry-After" in res.headers:
        try:
            retry_after = int(res.headers["Retry-After"])
        except (ValueError, TypeError):
            pass
    
    # If we have a valid retry_after value, sleep and retry
    if retry_after and retry_after > 0:
        if self.verbose:
            print(f"Rate limit hit. Retrying after {retry_after} seconds...")
        time.sleep(retry_after)
        continue
    # If no valid retry_after header, use a default backoff
    elif error_retry(" (rate limited)"):
        continue