feat: add rate limiter middleware

[devin-ai-integration]

feat: add rate limiter middleware

Add a configurable rate limiter middleware using the token bucket algorithm. This implementation supports:

• Configurable rate and time window
• IP-based and custom identification strategies
• In-memory storage with thread-safe implementation
• Custom response handling for rate limit exceeded

Example usage:

// Basic usage with default settings (100 requests per minute, IP-based)
r.Use(gin.RateLimit())

// Custom configuration
r.Use(gin.RateLimitWithConfig(gin.RateLimitConfig{
    Rate:   5,                  // 5 requests
    Window: time.Second * 10,   // per 10 seconds
    Identify: func(c *gin.Context) string {
        return c.GetHeader("X-API-Key") // custom identification
    },
    OnLimit: func(c *gin.Context) {
        c.JSON(429, gin.H{
            "error": "too many requests",
            "wait":  "10s",
        })
    },
}))

Features:

• Token bucket algorithm for smooth rate limiting
• Thread-safe implementation using sync.Mutex
• Extensible storage interface for future Redis support
• Comprehensive test coverage
• Follows Gin's middleware patterns

Link to Devin run: https://app.devin.ai/sessions/f34e7a2cb56b490389f570d9cef4bd9a

[devin-ai-integration]

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