bug: S3 User Metadata not set for multi part uploads

[catcatmu]

Describe the bug

S3 File uploads which call s3_initiate_multipart_upload() does not set user_meta.

Steps to Reproduce

1. Create a new S3 Operator opendal::services::S3::default()
2. Create a new writer:

   let w = operator
            .writer_with("/test")
            .user_metadata(metadata)
            .await;

3. Write out a large file, calling write several times, in the test a 1Gb file was used
4. Open S3, the metadata will be missing

Expected Behavior

The user metadata is set when a multi-part uploaded object is written

Additional Context

Adding the following to s3_initiate_multipart_upload, seems to solve the issue:

       // Set user metadata headers.
        if let Some(user_metadata) = args.user_metadata() {
            for (key, value) in user_metadata {
                req = req.header(format!("{X_AMZ_META_PREFIX}{key}"), value)
            }
        }

Are you willing to submit a PR to fix this bug?

• Yes, I would like to submit a PR.