feat(ratelimiter): add FixedWindow limiter and tests (#196)

* feat(ratelimiter): add FixedWindow limiter and tests

Introduce a FixedWindow rate limiter implementation, allowing a fixed number
of events per interval. Add comprehensive unit tests covering core behavior,
parameter validation, setters, concurrency, and edge cases.

* docs: update README to include Fixed Window rate limiter details

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Co-authored-by: Kashif Khan <70996046+kashifkhan0771@users.noreply.github.com>

Author: kyriakid1s