Move to a callback-based sftp server implementation

**What are you trying to do?**
When removing the libssh deprecation warnings, the sftp server initialization suggested to move to the callbacks-based server implementation. After some research, I have found in Gitlab the following libssh mirror that does implement an sample_sftpserver.c: https://gitlab.com/libssh/libssh-mirror/-/blob/master/examples/sample_sftpserver.c And in the PR they did they comment more on it.
I am unsure if this is a fork of our libssh or viceversa, but I think it would be nice to have this example.
Original PR that created example:
https://gitlab.com/libssh/libssh-mirror/-/merge_requests/263/diffs#cadf2ffbd25228d28ef40122660aa3c8ffb7d0d4
Merged PR:
https://gitlab.com/libssh/libssh-mirror/-/merge_requests/340/diffs

**What's your proposed solution?**
Modernize/refactor the sftp server implementation to be callback based.