Add SSH protocol adapter

[tschak909]

Now that libssh2 is embedded, a protocol adapter can be made to be able to do interactive shell sessions:

To implement:

[ ] copy https://github.com/FujiNetWIFI/fujinet-platformio/blob/master/lib/network-protocol/Test.cpp and https://github.com/FujiNetWIFI/fujinet-platformio/blob/master/lib/network-protocol/Test.h to SSH.cpp and SSH.h in lib/network-protocol [ ] implement functions (open, close, read, write, status) [ ] use netcat (e.g. on atari-apps/Networking TNFS) to test connection with a host

Existing protocol code can be seen from the above two links, as well as https://github.com/FujiNetWIFI/fujinet-platformio/blob/master/lib/network-protocol/TCP.cpp and https://github.com/FujiNetWIFI/fujinet-platformio/blob/master/lib/network-protocol/TCP.h

In addition, there is plenty of example code with libssh2, here is a complete ssh2 client that opens an interactive terminal: https://www.libssh2.org/examples/ssh2.html