Support for hex_digits_at_end SHA1 during upload

[kothar]

B2 supports adding the SHA1 hash of an uploaded file at the end of the content stream instead of in the headers:

https://www.backblaze.com/b2/docs/uploading.html

The advantage of this is that you can upload a file and calculate the SHA1 without needing to pre-process the file data before uploading to compute the SHA1.