Add Manage group access tokens custom permission

What does this MR do and why?

This MR adds 'Manage group access tokens' as a customizable permission, so that it can be added onto any base role.

This makes managing group access tokens available to all roles, as the Owner role might have too many privileges for all use cases.

Issue: #428353 (closed)

Screenshots or screen recordings

Adding the permission as group owner	Group member with custom role

How to set up and validate locally

1. Create a namespace and a project within that namespace and apply the Ultimate license to the namespace.
2. Create a new custom role that enables Manage group access tokens from Group -> Settings -> Roles and Permissions
3. Invite a user to the group and select the new custom role.
4. Log in as the user with the custom permission and check if you can:
   • Manage access tokens from the UI
   • Manage acces tokens via REST API

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.