Add "GET /groups/:id/enterprise_users" API endpoint

What does this MR do and why?

This MR adds "GET /groups/:id/enterprise_users" API endpoint that allows top-level group Owners to get list of their enterprise users and see their private attributes, like email, two_factor_enabled, etc.

See the following issue for more details #438366 (closed).

References

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• #438366 (closed)

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Before	After

How to set up and validate locally

1. Make sure the GitLab instance simulates or a SaaS instance since Enterprise Users is a SaaS feature
2. Configure "Automatic claims of enterprise users". For testing purposes on the local environment you can claim the user manually from the Rails console:

User.find_by_username('USERNAME').user_detail.update(enterprise_group_id: GROUP_ID)

3. Request the API endpoint as a group Owner. The enterprise user should be listed in the response with attributes as per the docs.

• curl --header "PRIVATE-TOKEN: <your_access_token>" https://gitlab.example.com/api/v4/groups/GROUP_ID/enterprise_users

doc/api/group_enterprise_users.md

+---
+stage: Govern
+group: Authentication
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://handbook.gitlab.com/handbook/product/ux/technical-writing/#assignments
+---
+
+# Group enterprise users API
+
+DETAILS:
+**Tier:** Premium, Ultimate
+**Offering:** GitLab.com
+
+Interact with [enterprise users](../user/enterprise_user/index.md) using the REST API.
+
+These API endpoints only work for top-level groups. Users do not have to be members of the group.
+
+Prerequisites:
+
+- You must have the Owner role in the group.
+
+## List enterprise users
+
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/438366) in GitLab 17.7.
+
+Get a list of enterprise users of a given group.
+
+```plaintext
+GET /groups/:id/enterprise_users
+```
+
+Parameters:
+
+| Attribute        | Type           | Required | Description |
+|:-----------------|:---------------|:---------|:------------|
+| `id`             | integer/string | yes      | ID or [URL-encoded path of the group](rest/index.md#namespaced-paths) |
+| `username`       | string         | no       | Return single user with a specific username |
+| `search`         | string         | no       | Search users by name, email, username |
+| `active`         | boolean        | no       | Return only active users |
+| `blocked`        | boolean        | no       | Return only blocked users |
+| `created_after`  | datetime       | no       | Return users created after the specified time |
+| `created_before` | datetime       | no       | Return users created before the specified time |
+| `two_factor`     | string         | no       | Filter users by Two-factor authentication. Filter values are `enabled` or `disabled`. By default it returns all users |
+
+Example request:
+
+```shell
+curl --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/groups/:id/enterprise_users"
+```
+
+Example response:
+
+```json
+[
+  {
+    "id": 66,
+    "username": "user22",
+    "name": "John Doe22",
+    "state": "active",
+    "avatar_url": "https://www.gravatar.com/avatar/xxx?s=80&d=identicon",
+    "web_url": "http://my.gitlab.com/user22",
+    "created_at": "2021-09-10T12:48:22.381Z",
+    "bio": "",
+    "location": null,
+    "public_email": "",
+    "skype": "",
+    "linkedin": "",
+    "twitter": "",
+    "website_url": "",
+    "organization": null,
+    "job_title": "",
+    "pronouns": null,
+    "bot": false,
+    "work_information": null,
+    "followers": 0,
+    "following": 0,
+    "local_time": null,
+    "last_sign_in_at": null,
+    "confirmed_at": "2021-09-10T12:48:22.330Z",
+    "last_activity_on": null,
+    "email": "user22@example.org",
+    "theme_id": 1,
+    "color_scheme_id": 1,
+    "projects_limit": 100000,
+    "current_sign_in_at": null,
+    "identities": [
+      {
+        "provider": "group_saml",
+        "extern_uid": "2435223452345",
+        "saml_provider_id": 1
+      }
+    ],
+    "can_create_group": true,
+    "can_create_project": true,
+    "two_factor_enabled": false,
+    "external": false,
+    "private_profile": false,
+    "commit_email": "user22@example.org",
+    "shared_runners_minutes_limit": null,
+    "extra_shared_runners_minutes_limit": null,
+    "scim_identities": [
+      {
+        "extern_uid": "2435223452345",
+        "group_id": 1,
+        "active": true
+      }
+    ]
+  },
+  ...
+]
+```