feat: add per-endpoint rate limiting middleware

[Iaguiz]

Adds EndpointRateLimitMiddleware for configurable per-endpoint rate limiting.

How to use:

class BookEndpoint(RestEndpoint, HttpMethod.GET):
    title: str = Field(min_length=1)

    class Meta:
        rate_limit = {"requests": 3, "window": 60}

Returns 429 Too Many Requests with X-RateLimit-* headers when the limit is exceeded.

Includes example (examples/19_rate_limit.py) and tests (tests/test_rate_limit_middleware.py).

To test:
pytest tests/test_rate_limit_middleware.py -v