Add generic class for handling methods and subscriptions for DDP APIand factor out authentication logic using this this mechanism.

Summary:

• Add generic class for handling methods and subscriptions for DDP API
• Add utility function to convert strings to QJsonObject/QJsonArray
• Factor out authentication logic in its own class (DDP API)
• Add tests for DDPAuthenticationManager

Reviewers: mlaurent, dfaure, kfunk

Reviewed By: mlaurent

Differential Revision: https://phabricator.kde.org/D29382