Feature: Expose ETag of uploaded object

[hashbrowncipher]

When completing a multipart upload, S3 offers the client an ETag which it can use to verify the uploaded object. s3gof3r currently verifies that the ETag matches its expectations and then swallows the value. If it instead bubbled up the returned ETag to the caller, the caller could store the ETag as metadata along with the object key and verify the object hasn't changed during a subsequent GET/HEAD request.