Add SFTPFileTransfer

[jl-wynen]

Giving users full SSH access is dangerous, so at ESS, we won't use this in production. Instead, we may use SFTP. And we definitely use SFTP for an upcoming summer school.
So this PR implements a new file transfer that uses only SFTP. It is mostly a copy of SSHFileTransfer but with replacements for all SSH-specific functions.

We lose some functionality. In particular:

• We cannot determine the timezone of the server. This is mitigated by using the upload time on the local machine in UTC as the file creation time. This is equivalent to the old method up to a small delay.
• We currently cannot validate uploaded files. This requires computing checksums on the server. There is an extension to SFTP that can do this. But the test server and current ESS servers do not implement it.

[jl-wynen]

Note https://stackoverflow.com/questions/30056566/how-to-perform-checksums-during-a-sftp-file-transfer-for-data-integrity

[jl-wynen]

The build fails because it references a URL that will only exist once this PR has been merged. (A code blob on main)