Pure-PHP ssh-agent client identity object
Instantiation should only be performed by \phpseclib3\System\SSH\Agent class. This could be thought of as implementing an interface that phpseclib3\Crypt\RSA implements. ie. maybe a Net_SSH_Auth_PublicKey interface or something. The methods in this interface would be getPublicKey and sign since those are the methods phpseclib looks for to perform public key authentication.