Support AppendBlobs and AppendBlock operation

[joaocc]

Hi, These seem to be supported on API version 2015-02-21 and later.

• https://docs.microsoft.com/en-us/rest/api/storageservices/understanding-block-blobs--append-blobs--and-page-blobs#about-append-blobs
• https://docs.microsoft.com/en-us/rest/api/storageservices/append-block

It would be great if this new method could be added. Thx