Add rate limiter exercise

[andreatanky]

Exercise idea
Implement a fixed-window rate limiter that enforces a maximum number of actions per key within a time window.

Concepts taught
State management: tracking per-key counters and window start times.
Time handling: reasoning about elapsed time and window boundaries.
Boundary conditions: handling “at the exact moment” vs “just past” a window.

Link to discussion in forum: https://forum.exercism.org/t/new-exercise-proposal-rate-limit/19109/10