Add support for fixed interval refill

In opposite to smooth refill which adds token as soon as possible, interval refill should regenerate tokens periodically.

Proposed API:

public static Refill fixedInterval(long tokens, Duration period) {
    // ...
}

public static Refill fixedInterval(Instant timeOfFirstRefill, long tokens, Duration period) {
     // ...
}

[vladimir-bukhtoyarov]

The proposed API to specifying interval refill:

// 1000 tokens per minute, refill happens with one second interval
Bandwidth limit = Bandwidth.simple(1000, Duration.ofMinutes(1))
                                .withFixedRefillInterval(Duration.ofSecond(1));

Bucket bucket = Bucket4j.builder()
    .addBandwith(limit)
    .build();

I am not sure that withFixedRefillInterval is best name for method(resilience4j uses limitRefreshPeriod), feel free to propose more suitable.

[vladimir-bukhtoyarov]

released https://github.com/vladimir-bukhtoyarov/bucket4j/releases/tag/4.0.0