Write retry backoff code

Create RetryBackoff.java. Assume Java 17.

Implement:

Duration backoff(Config config, int attempt, HttpResponse<?> response)

Rules:

• If response is not null and has a valid Retry-After header, return that duration.
• A valid Retry-After value is a positive whole number of seconds.
• If the header is missing, blank, zero, negative, or not a number, return the exponential backoff.
• The exponential backoff is config.baseDelay().multipliedBy(1L << Math.min(attempt - 1, 8)).
• Include Optional<Duration> parseRetryAfter(String value).
• Include this nested record:

record Config(Duration baseDelay) {}