Combine authentication methods into one method

[orenbm]

The public API for authentication is currently separated into 2 parts:

1. Authenticate
2. ParseAuthenticationResponse

Consumers of the authn-client library currently need to first call the Authenticate method, get the output and call the method ParseAuthenticationResponse with it. This adds redundant complexity to the flow and can be reduced easily by letting the Authenticate method perform the whole flow by itself, so that consumers can call one method without the need of another.

So, instead of performing the following:

resp, err := authn.Authenticate()
if err != nil {
	// Do something
}

err = authn.ParseAuthenticationResponse(resp)
if err != nil {
	// Do something
}

consumers can perform the following:

err := authn.Authenticate()
if err != nil {
	// Do something
}