Add SftpFs beta backend

[xor-gate]

As discussed in #47, have fun :-)

[CLAassistant]

All committers have accepted the CLA.

[xor-gate]

It seems continues integration chokes on the test, i have no real clue how to make this actually work. The problem is we need to spawn a sftp server and test against it. Or remove the "example/test" completely.

[xor-gate]

@mbertschler you are right I fixed up the test and now spawn a Sftp server and generate the RSA host key 👍. Still the implementation and the test needs some love and polishing but it is nice to have a stub sftp server and don't rely on a hosted sftp server.

[mbertschler]

@xor-gate the Sftp server in the tests is a good idea 👍.
I have not tested it yet but I think this is ready to merge as an experimental feature. I will probably play around with it in the next days, and also see if this FS works with our existing tests.

[xor-gate]

Thank you @mbertschler. And yes having the sftp server stub in the test makes sure with the CI tests that everything is still functional.