Support JSch IdentityRepository for authentication

[cfieber]

This allows use of jsch-agent-proxy to pull identities from ssh-agent.

As a workaround, the below example works, but feels pretty brittle for two reason:

1. taking advantage of Groovy letting me ignore the private modifier on engine.jsch
2. passing defaultPassword: '' because the connect() method expects either a password or a keyFile

def identityRepository = new RemoteIdentityRepository(ConnectorFactory.default.createConnector())
def engine = new SshDslEngine(new SshOptions(defaultPassword: '', jschProperties: [(SshDslEngine.SSH_PREFERRED_AUTHENTICATIONS): 'publickey']))
    engine.jsch.setIdentityRepository(identityRepository)