[OAuth] Support private URI scheme redirection (Android, iOS)

There are broadly speaking three categories of redirect_uris for native applications

1) loopback URIs (http://127.0.0.1:2344/oauth2redirect, currently supported by qtnetworkauth via QOAuthHttpServerReplyHandler)
2) private URI-schemes (com.myapp.com:/oauth2redirect, what this Jira task is about)
3) claimed "https" scheme URIs (https://my.app.com/oauth2redirect

This task is about 2), adding convenience support for the private URI schemes. This most probably means implementing a new ReplyHandler class into the qtnetworkauth module (QOAuthUriSchemeReplyHandler). Also, the documentation must include what the application developers need to do with regards to application manifests (Android) and entitlements (iOS).

This task relates closely to adding support for "claimed https URI schemes". They will in all likelihood share the same convenience class.
Automatic CI testing is likely going to be somewhat limited as the custom URIs may invoke a dialogue for the user to select an application.