Provide client session connection context that is propagated to the SSH session

When creating a client session the caller cannot provide any kind of "context" that will be propagated to the session, and thru it to the various listeners/verifiers/authenticators. Such a context may be useful in the case where an SshClient instance is used to connect various hosts or the same host for various purposes. In such a case, it is conceivable that the user may wish the various registered "callbacks" to behave differently according to some connection context provided when the SSH session was established. The current code allows such decisions based on the peer host address/port, but this information might not be enough for the required decision functionality.