[libssh clients only] Allow certificate data and type to be provided instead of certificate file

[pkittenis]

Is your feature request related to a problem? Please describe.
Certificate authentication does not support certificate data. A certificate file must be provided

Describe the solution you'd like
Allow certificate data and certificate type to be specified programmatically without a file.

Describe alternatives you've considered
Writing certificate data to temporary file with correct certificate header.

Additional context
This is specific to the pssh.clients.ssh clients.