Missing `shared_with_groups` property on Project and Group models

[JasonMouwen]

Description

The GitLab REST API returns a shared_with_groups array on both the
project (GET /projects/:id) and group (GET /groups/:id) responses,
but this property is not mapped in NGitLab.Models.Project or
NGitLab.Models.Group. As a result, consumers can't see which groups a
project/group has been shared with.

API reference

• Projects API: https://docs.gitlab.com/api/projects/
• Groups API: https://docs.gitlab.com/api/groups/

Example payload

"shared_with_groups": [
  {
    "group_id": 4,
    "group_name": "Twitter",
    "group_full_path": "twitter",
    "group_access_level": 30,
    "expires_at": null
  }
]

Proposed change

• Add a new model type (e.g. SharedWithGroup) mapping group_id,
  group_name, group_full_path, group_access_level and expires_at.
• Add [JsonPropertyName("shared_with_groups")] public SharedWithGroup[] SharedWithGroups { get; set; }
  to both Project and Group.
• Mirror this in NGitLab.Mock and add tests.