refactor(util): replace header redaction with allowlist in logs

- Switch header logging from redacting sensitive headers to allowlisting
  key headers (`range`, `user-agent`, `host`, `x-request-id`)
- Remove `RedactedHeaderMap`; introduce `KeyRequestHeader` that filters
  instead of scrubbing
- Omit bulky/low-value `Download` fields (`certificate_chain`,
  `content_for_calculating_task_id`, etc.) from debug output
- Use `finish_non_exhaustive` to signal intentional field omission
- Update doc comments and tests to reflect the new allowlist approach

Signed-off-by: Gaius <gaius.qi@gmail.com>