Include support for viewing and changing the default authentication method in Graph API

Right now, the Graph API can list the Authentication methods configured for a user, but does not return which the user has configured as the default and does not allow setting of the default method. (And, even as the tenant global admin, you cannot view this through the Azure portal.)

This causes the helpdesk extra work. When configuring the Authenticator app as a user, you can create an auth method simply by signing into the app, but it doesn't give you the option of setting or changing your default method. So the helpdesk ends up having to talk people through signing into the My Security Info portal, and setting the default method that way. Would be much easier if our helpdesk app could shows that the user's managed to configure the right auth methods, then go in and set the default for them.