Add a mean to set headers on authenticated network requests

QAbstractOAuth2::get() can be used to send authenticated request, but it does not provide any mean to set custom headers. Such headers may be required for web API that require a header with the API Key.

I see 2 ways to solve this:

• Add a get() function that takes a `QNetworkRequest` parameter so that the user can set its headers beforehand.
• Add a function that "authenticates" a `QNetworkRequest` and then the user can send it with its own network access manager.

The current alternative is to get the token from QAbstractOAuth2 and manually add the "Bearer" header. On a side note, the current API does not provide a mean to retrieve the token type, so we cannot even know if "Bearer" is the right protocol.