Parallel buffer upload API?

We are evaluating this library (vs azure-storage-cpp), and the smaller dependency tree is appealing. One API that seems to be missing is a parallel put from a buffer/stream of known size. It looks like the `blob_client::upload_file_to_blob` function could be refactored to take a buffer, or a similar function written which splits over the buffer and then finalizes the block list like `upload_file_to_blob` does.

Is this an API you would consider adding, or did I miss an existing API? Thanks!

cc @joe-maley @Shelnutt2