feat: configure resumable HTTP retry and validation behavior

[digizeph]

Context

ResumableHttpReader uses a fixed retry policy: five connection attempts with a 200 ms exponential backoff. It also accepts a resumed resource optimistically when the original response provides neither ETag nor Last-Modified.

Those defaults are appropriate for the convenience helper, but long-running readers and authenticated/private HTTP endpoints may need explicit control and visibility.

Proposed API

Add a public options type, for example:

ResumableHttpOptions {
    max_retries: 5,
    base_retry_delay: Duration::from_millis(200),
    max_retry_delay: ...,
    validator_policy: ValidatorPolicy::AllowMissing,
}

Expose it through a method such as:

OneIo::get_resumable_http_reader_with_options(url, options)

Keep get_resumable_http_reader as the convenience API and preserve its existing behavior by using the default options.

Scope

• Configure retry count and backoff.
• Choose whether a resume requires a response validator.
• Provide optional resume-attempt observability (for example, a callback or structured log event).
• Retain Accept-Encoding: identity as a fixed correctness requirement for range requests.

Acceptance criteria

• Existing callers retain the current retry and validation behavior without source changes.
• Callers can configure retry limits and delays per reader.
• Strict validation can reject resources without a usable ETag or Last-Modified.
• Tests cover default compatibility and each non-default option.