Wire up the new Device Flow feature

[olivierdagenais]

OAuth 2.0 Device Flow is now wired up and will be attempted as a last resort.

Manual testing

1. Add a Git remote to a clone of this repo pointing to a VSTS account that uses Microsoft Accounts (MSA). Let's call the remote msarepo.

2. Run git push msarepo.

3. When the JavaFX browser opens, immediately close the window. This used to completely abort the GCM4ML; now we get something like this:

   ------------------------------------
   OAuth 2.0 Device Flow authentication
   ------------------------------------
   To complete the authentication process, please open a web browser and visit the following URI:
   https://aka.ms/devicelogin
   When prompted, enter the following code:
   E3P3CAVVR
   Once authenticated and authorized, execution will continue.
   

4. Follow the instructions to authenticate.

5. The push succeeds.

6. Add a Git remote to a clone of this repo pointing to a VSTS account that uses Azure Active Directory (AAD). Let's call the remote aadrepo.

7. Run git push aadrepo.

8. When the JavaFX browser opens, immediately close the window. This used to completely abort the GCM4ML; now we get something like this:

   ------------------------------------
   OAuth 2.0 Device Flow authentication
   ------------------------------------
   To complete the authentication process, please open a web browser and visit the following URI:
   https://aka.ms/devicelogin
   When prompted, enter the following code:
   EYCSHYPEC
   Once authenticated and authorized, execution will continue.
   

9. Follow the instructions to authenticate.

10. The push succeeds.

Mission accomplished!