Google provider: limit requested OAuth scopes

Summary:
Limit the scopes to what is actually permitted in the Google App settings:
contacts and calendars for future PIM integration, GDrive for KIO-GDrive,
and Youtube (upload-only) for the Purpose sharing plugin. We can extend
this in the future if needed easilly, it's easier for us to get the
Google App verified if we can proof and show how the individual scopes
are used by KDE.

Reviewers: elvisangelaccio, bshah

Reviewed By: elvisangelaccio, bshah

Differential Revision: https://phabricator.kde.org/D26454