Add Redis-backed Rate Limiter (.NET 8+)

[imperugo]

Summary

Implement `System.Threading.RateLimiting.RateLimiter` backed by Redis for distributed rate limiting across multiple app instances.

Problem

.NET 8's `RateLimiting` middleware is in-memory by default. In a multi-instance deployment, each instance has its own counter — so a limit of 100 req/min becomes 100×N. Redis-backed rate limiting solves this.

Proposed Algorithms

Fixed Window

```csharp
services.AddRedisFixedWindowRateLimiter("api", options =>
{
options.Window = TimeSpan.FromMinutes(1);
options.PermitLimit = 100;
});
```

Sliding Window

```csharp
services.AddRedisSlidingWindowRateLimiter("api", options =>
{
options.Window = TimeSpan.FromMinutes(1);
options.PermitLimit = 100;
options.SegmentsPerWindow = 6;
});
```

Implementation

• Use `StringIncrementAsync` with TTL for fixed window
• Use Lua script for sliding window (atomic check-and-increment)
• Consider a separate package: `StackExchange.Redis.Extensions.RateLimiting`

Considerations

• SE.Redis 2.12 has `StringGcraRateLimitAsync` — a built-in GCRA algorithm
• Depends on Add StringIncrementAsync and StringDecrementAsync #634 (StringIncrementAsync)