[YuangGao]

Closes #4290

Rationale for this change

• A REST server can advertise idempotency-key-lifetime, after which clients send an Idempotency-Key (UUIDv7) on mutations and reuse it on retry, so a retried commit can't apply twice. PyIceberg sent no key and never retried POST.
• With the lifetime advertised, each mutation now carries a fresh key, and the rest.client.* retry policy also covers POST, reusing the key across retries, as Java's RESTSessionCatalog does.
• The lifetime itself isn't enforced: urllib3 retries are bounded and finish well within typical lifetimes.

Are these changes tested?

• test_rest.py — mutations carry a UUIDv7 key only when advertised, a different one per operation, and reads carry none; a POST retried after a 503 reuses its key.

Are there any user-facing changes?

• Against servers that advertise idempotency-key-lifetime, mutations send Idempotency-Key and POST is retried under rest.client.*.