Add built-in rate limiting support

[tw4]

Summary

Nexar has no built-in rate limiting mechanism. When integrating with third-party APIs that enforce rate limits, users must implement throttling logic themselves.

Use Cases

• APIs with requests-per-second or requests-per-minute limits
• Preventing 429 Too Many Requests errors
• Controlled burst traffic scenarios

Proposed Solution

Add rate limiting options to NexarConfig using .NET 7+'s built-in System.Threading.RateLimiting primitives.

var config = new NexarConfig
{
    RateLimit = new RateLimitOptions
    {
        RequestsPerSecond = 10,
        BurstSize = 5
    }
};

Alternatively, support a RateLimiter injection point so users can plug in their own limiter (token bucket, sliding window, etc.).