fix: update aws-sigv4 to 1.x and secrecy to 0.10

Fixes compilation errors introduced by the aws-sigv4 major version bump
from 0.3.1 to 1.x (as required by dependabot PR JanKaul#320).

Key changes:
- Replace `SigningParams::builder()` (now an enum) with `v4::SigningParams::builder()`
  using `aws_credential_types::Credentials` / `Identity`
- Replace `SignableRequest::from(&http_request)` with `SignableRequest::new()`
- Replace `take_headers()` with `headers()` on `SigningInstructions`
- Replace `aws_sigv4::http_request::Error` with `SigningError`
- Update secrecy from 0.8 to 0.10 (SecretString now exposes &str, not &String)
- Remove now-unused `http` crate dependency

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: JanKaul