Support "/namespaces" API

[twindan]

The /namespaces API (https://docs.gitlab.com/ee/api/namespaces.html) returns a list of all namespaces available to a user.

It seems like the type returned from that API is compatable with GitLabApiClient.Models.Projects.Responses.Namespace.

[reeloadead]

Current GET https://gitlab.com/api/v4/namespaces response has more fields:

    {
        "id": 00000,
        "name": "tester",
        "path": "tester",
        "kind": "user",
        "full_path": "tester",
        "parent_id": null,
        "avatar_url": "https://secure.gravatar.com/avatar/***",
        "web_url": "https://gitlab.com/tester",
        "billable_members_count": 1,
        "seats_in_use": 1,
        "max_seats_used": 0,
        "plan": "free",
        "trial_ends_on": null,
        "trial": false
    }

• maybe it worth to implement support for /namespace API or at least
• remove sealed modifier from response types like https://github.com/nmklotas/GitLabApiClient/blob/master/src/GitLabApiClient/Models/Projects/Responses/Namespace.cs