Return response bytes from `ssh.sshUploadPack()`

This MR complements Resolve "GitLab sshd should include data transf... (gitlab-shell!831 - merged) by returning stdoutCounter.N from ssh.sshUploadPack() which is then returned from ssh.SSHUploadPackWithSidechannel() as part of gitalypb.SSHUploadPackWithSidechannelResponse.

The goal is to return the number of bytes to gitlab-shell so we can log this information within the gitlab-shell logs along with username and project details for monitoring purposes.

Related:

• GitLab sshd should include data transfer bytes ... (gitlab-shell#648 - closed)
• Resolve "GitLab sshd should include data transf... (gitlab-shell!831 - merged)