Support Harbor integration properties in API response

I am working on the Terraform provider validation for Harbor, and I found that the library is currently unable to read the integration configuration back from the API.

When I query the GitLab API for the Harbor integration settings, the JSON response includes a section with the URL, project name, and username. However, the current library implementation does not read this section of the response, so these values are lost and cannot be accessed by consumers like Terraform.

We need the library to be updated to parse this specific section from the API response so that we can verify the current configuration state.

Reproduction: 

GET /api/v4/groups/<group_id>/integrations/harbor

Actual API Response:

{

"id": 45,
"title": "Harbor",
"active": true,
"...": "...",

"properties": {

    "url": "https://demo.goharbor.io",
    "project_name": "library",
    "username": "testuser"
    
  }

}