Server: Migration to the new callback-based server implementation in libssh

[artyom-poptsov]

The following procedures are deprecated in libssh in favor of the new callback-based server implementation:

• ssh_message_auth_password
• ssh_message_auth_pubkey
• ssh_message_auth_publickey_state
• ssh_message_channel_request_pty_term
• ssh_message_channel_request_pty_width
• ssh_message_channel_request_pty_height
• ssh_message_channel_request_pty_pxheight
• ssh_message_channel_request_pty_pxwidth

We should deprecate this API as well and provide a proper callback-based server.

See https://api.libssh.org/master/group__libssh__server.html