Enhancement: allow customizing MaxConcurrentRequestsPerFile and MaxPacket parameters in the sftp backend

SFTP over long fat links suffers from poor performance due to its by default
overly small max payload size. But implementations such as OpenSSH tend to
accept vastly bigger packets, which improves backup performance for this
kind of situation. Restic now allows customizing the max packet size and
max concurrent requests per file parameters in the sftp backend.

https://github.com/restic/restic/pull/4445