Include OAuth url in data source retrieval

[RubenFricke]

Describe the bug
When the data sources are retrieved the OAuth URL should be retrieved as well. We will migrate the OAuth flow partially from backend to frontend and for doing this, the frontend should be able to use OAuth Url. To keep everything scalable and dynamically, this still should come from the backend.

To Reproduce
Steps to reproduce the behavior:

1. Retrieve one or multiple data sources
2. The OAuth URL property should get added

Expected behavior
The OAuth URL property should get returned.