Print AWS SigV4 headers in verbose stderr output

[ryanfowler]

Summary

• Sign AWS-authenticated requests before verbose request metadata is printed so stderr shows the final outgoing headers.
• Add an integration regression that checks -vv output includes the AWS SigV4 authorization, date, session token, and payload hash headers.

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo test --all-features --test integration -- --test-threads=1