[v14] backoffice user login providers endpoint

[Migaroez]

Prerequisites

• I have added steps to test this contribution in the description below

Description

Added an endpoint to get a all available external login providers with status for the current user to show whether it is linked or not.

During tests and pair review, it was noted the methods in the backofficecontroller had to much logic, so I refactored methods that had to do with linking/unlinking into a separate service as this logic was part of the test path for setting up the new endpoint.

Added unit tests for the new service

Testing

• You can still unlink providers as described in V14 External login linking + Proposed error handling #16052
• Linking now requires you to 'preauthenticate' the endpoint by calling GET https://localhost:44339/umbraco/management/api/v1/security/back-office/link-login-key?provider=yourprovider and then to use the obtained key in the actual link call
• Using the steps above, you should be able to manipulate the output of the new endpoint GET:/umbraco/management/api/v1/user/current/login-providers

[kjac]

Code looks good 👍