Create read_admin_subscription custom admin ability

What does this MR do and why?

Related to #507961 (closed)

This MR adds the custom ability for a non-admin user to read the instance subscription details in the admin dashboard.

References

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MR acceptance checklist

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How to set up and validate locally

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1. Enable the feature flag.

   Feature.enable(:custom_ability_read_admin_subscription) 

2. Create the admin role.

   admin_member_role = MemberRole.create(name: 'View subscription', description: 'View subscription', read_admin_subscription: true)

3. Assign the member role to a user

   Users::UserMemberRole.create(member_role: admin_member_role, user: user)

4. Log in with the user account and navigate to /admin/subscription. You should see the subscription details but not have any access to other admin functionality.