Streaming multipart uploads drop custom headers at CHUNK_SIZE

[mfroembgen]

Describe the bug

put_object_stream_builder applies per-upload metadata and custom headers when an upload uses single-part PutObject. When the payload reaches CHUNK_SIZE and switches to multipart upload, those headers are not sent on CreateMultipartUpload.

S3-compatible services require completed-object configuration such as user metadata, cache control, storage class, and SSE-S3/KMS headers during multipart initiation. The upload succeeds, but the completed object silently loses that configuration.

To Reproduce

1. Start an S3-compatible MinIO instance and create a bucket.
2. Upload synthetic byte payloads using put_object_stream_builder with custom metadata.
3. Check the completed object's metadata with HEAD.

Payload size	Upload path	Metadata
CHUNK_SIZE - 1	single-part	present
CHUNK_SIZE	multipart	missing
above CHUNK_SIZE	multipart	missing

Expected behavior

Streaming-builder headers should be signed and sent on CreateMultipartUpload, so completed-object metadata and other initiation-time configuration survive at and above the multipart boundary.

Environment

• Rust: 1.97.1
• rust-s3: 0.37.2
• Storage: MinIO-compatible S3 endpoint

Additional context

This is the multipart-specific gap related to #408. A previous implementation direction was discussed in #446. The consumer-visible reproduction is brunojppb/turbo-cache-server#620, and the proposed fix is #471.