Added method to get current capacity of rate limiter for given algorithm

[norberttech]

This PR makes something I would call the final API shape

    /**
     * Record next hit, throws an extension where there are no available hits left according to the selected algorithm.
     *
     * @throws RateLimitException
     */
    public function hit(string $id) : void

    /**
     * Estimate time required to the next hit. If current capacity is greater than 0, time will be 0.
     */
    public function estimate(string $id) : TimeUnit

    /**
     * Returns current capacity according to the selected algorithm, when there are no available hits left, it will return 0.
     * Use RateLimiter::estimate method to find out when next hit will be possible.
     */
    public function capacity(string $id) : int

    /**
     * Try to record next hit, in case of rate limit exception take the cooldown time and sleep current process.
     */
    public function throttle(string $id, Process $process) : void