Optional client-side sliding-window rate limiter.

Use it to stay within the public detalji_subjekta limit of 6 requests/minute:

{:ok, _} = SudregEx.RateLimiter.start_link(name: :sudreg_limiter)
:ok = SudregEx.RateLimiter.acquire(:sudreg_limiter)
SudregEx.Api.detalji_subjekta(client, tip_identifikatora: "oib", identifikator: oib)

Not started by default — start one yourself (e.g. in your supervision tree). acquire/1 blocks the caller until a slot frees; the GenServer itself never sleeps, so admission decisions stay responsive.

Under many concurrent callers, waiters wake together and re-contend; admission stays correct (never more than limit per window) but a retry storm is possible — a non-issue for the single-consumer detalji_subjekta case.