Is your feature request related to a problem? Please describe.
Currently the SFTP related functions are under the SSH clients and also require an SFTP object be passed along with them.

This makes dealing with SFTP difficult, as an SFTP object first needs to be created then passed along to all the various SFTP operations.

Further more, some SFTP operations like mkdir, cd et al are not currently public.

Describe the solution you'd like
Create an SFTP client-like class to handle all SFTP operations in a more user friendly manner.

Under this new class, make all SFTP operations public and easy to use without requiring an SFTP object be passed into them.

Also make this class keep track of current working directory, to go with being able to change directory, make new directories and so forth.

Describe alternatives you've considered
Leaving SFTP as-is, which is less useful.

Additional context
See #117 and #394.