Authorize code - step by step to display the device code

[MSchmack]

In the authorize code method the identification & authorization is completed in one step which does not allow displaying the device code. Without the device code the user can not verify if the authentication is as intended (UX Issue).
This commit creates two methods that split the authorizeCode method in two. This allows first getting the response with the device code and then going for the authentication afterwards.

Example usage

                 final codeResponse =
                        await OpenIdConnect.authorizeDeviceGetDeviceCodeResponse(request: request);

                   // dsplay the user device code
                    setState(() {
                      deviceIdentification = codeResponse.userCode;
                    });

                    final response =
                        await OpenIdConnect.authorizeDeviceCompleteDeviceCodeResponseRequest(
                      request: request,
                      codeResponse: codeResponse,
                    );

Names could probably be chosen better :-)

[jhancock4d]

Both methods are valid. If the device code is pre-passed it's supposed to work like Microsoft device code authentication and thus not show it and just confirm the request.

But agreed that if the device code is not passed it should show it. Could you update your PR to support both approaches?