conn: add 'attach' to protocol handler, make libssh2 use it

conn: add 'attach' to protocol handler, make libssh2 use it

The libssh2 backend has SSH session associated with the connection but
the callback context is the easy handle, so when a connection gets
attached to a transfer, the protocol handler now allows for a custom
function to get used to set things up correctly.

Reported-by: Michael O'Farrell
Fixes #6898
Closes #7078