[feature]: support byte offset when retrieving a file

[mattgathu]

It would nice to be able to resume partial downloads by specifying the bytes offset from which to read from.

Python's ftplib supports this and can be a good reference source:

FTP.retrbinary(cmd, callback, blocksize=8192, rest=None)

Retrieve a file in binary transfer mode. [...] rest means the same thing as in the transfercmd() method.

FTP.transfercmd(cmd, rest=None) [...] If optional rest is given, a REST command is sent to the server, passing rest as an argument. rest is usually a byte offset into the requested file, telling the server to restart sending the file’s bytes at the requested offset, skipping over the initial bytes. .....