Support custom S3 conditional put headers

[Herrtian]

Closes #416.

Adds a custom-header mode for S3 conditional puts, so S3-compatible stores can wire PutMode::Create to headers such as x-oss-forbid-overwrite: true. Conflict responses continue to surface as AlreadyExists.

Checks:

• cargo fmt --check
• cargo test --features aws parse_s3_conditional_put_header --lib
• cargo test --features aws put_create_ --lib
• cargo clippy --features aws --lib -- -D warnings
• git diff --check