OAuth2 authorization code flow does not use the user-supplied user agent

Setting the user agent in a `QOAuth2AuthorizationCodeFlow` object does not affect the QNetworkRequest objects created and sent by the `requestAccessToken()` and `refreshAccessToken()` methods.  I was working with an OAuth provider who required the user agent to be set for these requests. Without the correct user agent, OAuth failed with a "Token request failed" warning issued by `_q_accessTokenRequestFailed()`.

At the time, I ended up rolling my own OAuth implementation with the HttpServer and Network modules. More recently–specifically after updating my codebase for 6.8.0–I revisited this issue. First, I forked Qt's oauth code. Then I changed `requestAccessToken()` and `refreshAccessToken()` to set the user agent based on the one set by the user. With that change, OAuth succeeded and I was able to receive tokens.

Would it be possible to modify QOAuth2AuthorizationCodeFlow to set the user agent in the request and refresh requests, or would this have unintended consequences?