[Backend] - create endpoint to get all oauth registration options

[R-Sandor]

Details

As the client options can be dynamically added the frontend should have a set of options displayed to the user.

Tasks

• To do this depends on having client-registration working successfully.
  - Verify that those issues are resolved first.
• https://stackoverflow.com/questions/79536603/how-can-i-retrieve-the-configured-oauth-registrations-in-springboot-oauth
  • Consider creating an in memory repository.
• Create endpoint /api/oauth2/providers