allow two-phase grant in QOAuth2AuthorizationCodeFlow

QOAuth2AuthorizationCodeFlow::grant() currently (5.9.1) gets an authorization code and then uses it to get an access token all in one step.

Could we have the option to authorize and return a code so that it can be stored and reused?

I suggest something like the following:

QOAuth2AuthorizationCodeFlow::authorize(), which would signal that a code is available.

QOAuth2AuthorizationCodeFlow::code() to return the code, so it can be stored, and QOAuth2AuthorizationCodeFlow::setCode(const QString&) to provides a previously retrieved authorization code.

QOAuth2AuthorizationCodeFlow::grantUsingCode(), which would skip the interaction with the user and just use a provided (setCode()) code to get an access token.