[Storage] upload roundtrip concerns with download API

[vincenttran-msft]

This is applicable to its current state, but will also still be applicable even after managed_download replaces download.

Against our current authoring, the only way to upload a downloaded blob is to buffer in-memory to be able to get it into a RequestContent:

 let download_response = source_blob_client.download(None).await?;
 let buffered: Bytes = download_response.into_body().collect().await?;
 // Only after full buffering can the data be passed to `upload`.
    dest_blob_client
        .upload(RequestContent::from(buffered.to_vec()), None)
        .await?;

Applying a similar workaround as @benbp :

    // content_length must be read from headers *before* deconstruct() consumes the response,
    // because the body stream itself has no len().
    let download_response = source_blob_client.download(None).await?;
    let content_length = download_response.content_length()?;
    let (_, _, body) = download_response.deconstruct();

 // use our custom SeekableStream adapter impl, but this makes retries impossible due to the always-error reset() impl
    let stream = DownloadBodySeekableStream {
        inner: Arc::new(Mutex::new(DownloadBodyInner {
            stream: Some(Box::pin(body)),
            remainder: Bytes::new(),
        })),
        len: content_length.unwrap_or(0),
    };

// But allows this to flow the bytes directly through instead of buffering in-memory
    dest_blob_client
        .upload(Body::SeekableStream(Box::new(stream)).into(), None)
        .await?;

[vincenttran-msft]

This is the crux of the issue.

[vincenttran-msft]

This is the crux of the workaround: a custom DownloadBodySeekableStream adapter so that we can get it into a RequestContent without collecting in-memory.