feat(credentials): Add support for listing all SaaS enterprise user personal access tokens

This change adds a new feature to the GitLab API client that allows administrators to view personal access tokens for users within a group. The implementation includes a new service called "GroupCredentials" that can retrieve a list of all personal access tokens associated with enterprise users in a top-level group, along with details like token names, creation dates, expiration dates, and usage information. The change also includes comprehensive tests to ensure the feature works correctly and handles error cases properly. This functionality helps administrators manage and audit security credentials across their organization's GitLab groups.