Add rate limiter problem specifications

[andreatanky]

Summary

• Introduces a new exercise that implements a fixed-window, per-key rate limiter with an injected monotonic time source.
• Adds canonical tests and specifications.

Details

1. Added: exercises/rate-limiter/canonical-data.json
   Property: rateLimiter
   Input: limit (int), windowMillis (int), operations (array)
   Operations:
   {"op":"allow","key":string} → produce a boolean in order
   {"op":"advance","millis":int} → advance the simulated clock
   Covers: within-window allowance, window reset, per-key independence, exact boundary semantics, multi-window behavior, and invalid configs (limit<=0, windowMillis<=0).

2. Added: exercises/rate-limiter/instructions.md
   Describes fixed-window semantics, per-key counters, boundary handling, and a suggested API with an injectable TimeSource to avoid sleeps in tests.

3. Added: exercises/rate-limiter/introduction.md
   Provides context and motivation for rate limiting.

4. Added: exercises/rate-limiter/metadata.toml
   Title and blurb for track integration.

Rationale

• Teaches a practical systems concept with clear determinism by using an injected time source.
• Fixed-window model keeps the exercise accessible and testable across tracks.

Discussion Link: https://forum.exercism.org/t/new-exercise-proposal-rate-limit/19109

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