Idempotency support

HTTP::API::Core supports API-specific idempotency keys without assuming that every API uses the same header name.

my $response = $api->post(
    '/payments',
    json => { amount => 1000 },
    idempotency => {
        key    => $request_key,
        header => 'Idempotency-Key',
    },
);

The option is intentionally explicit:

• key is the idempotency value.
• header is the exact API-specific header name.
• both must be non-empty scalar values.
• an explicitly supplied request header with the same case-insensitive name takes precedence.

The client does not generate keys automatically and does not make POST retryable merely because an idempotency key is present. Retry policy remains an explicit, separate concern.

This keeps the core service-neutral: APIs using Idempotency-Key, X-Idempotency-Key, or another convention can all use the same mechanism.