Align multipart transfer protocol with the protocol proposed in the official git-lfs repo

[shevron]

After we implemented multipart-basic support, I've started working with the Git LFS team on an official proposal to add a similar transfer protocol to Git LFS: https://github.com/git-lfs/git-lfs/pull/4438 . This is very similar, but slightly cleaned up and simplified version of our own protocol.

Once this proposal is accepted, it would be good to align our own implementation with it.

As it is not 100% compatible with our current transfer protocol, we can maintain BC by implementing it as a separate transfer adapter and keeping the old one around, but recommend the new one in our docs.