Add OAuth tokens to credentials inventory

What does this MR do and why?

This MR adds OAuth tokens to the credentials inventory, allowing administrators to view and revoke OAuth access tokens across their GitLab instance.

References

Fixes #526129

Screenshots or screen recordings

How to set up and validate locally

1. Navigate to Admin Area > Credentials (/admin/credentials)

2. Select OAuth tokens tab

3. Make sure that oauth tokens are listed and can be filtered

4. Navigate to Group > Secure > Credentials (/groups/your-group/-/security/credentials)

5. Make sure that oauth tokens are listed for the enterprise of that group and can be filtered

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.