[12.x] Add sliding window algorithm support to rate limiter

[SanderMuller]

Summary

Adds opt-in sliding window rate limiting to Laravel's rate limiter.
Fixed window limiting is susceptible to burst attacks at window boundaries.
A client can exhaust the limit at the end of one window and again at the start of the next, effectively doubling throughput in a short period.
Sliding window smooths this by weighting the previous window's count into the current calculation.

Formula

effectiveAttempts = floor(overlapRatio × previousWindowHits) + currentWindowHits

where overlapRatio = 1 - (elapsed / windowSize)

Usage

// Opt-in via fluent method on the existing Limit class
RateLimiter::for('api', function (Request $request) {
    return Limit::perMinute(60)
        ->by($request->user()?->id)
        ->slidingWindow();
});

Works with all existing Limit features: after() callbacks, response() callbacks, by() keys, and mixed limit arrays (fixed + sliding in the same limiter).