Make the /manage/personal_access_tokens GET endpoint use the credentials_inventory_pat_finder

• Related to #511922 (closed) & #511922 (closed)
• Blocked by !193375 (merged)

What does this MR do and why?

• Makes the groups/:id/manage/personal_access_tokens API use the new ::Authn::CredentialsInventoryPersonalAccessTokensFinder to handle performance issue
• Flagged behind credentials_inventory_pat_finder feature-flag

How to set up and validate locally

Top-level group owners should fetch only Enterprise Users on gitlab.com

Steps on GDK

• Create a top-level-group (save the group_id) and create a PAT with an API scope(save this), ssh_key, GrAT and PrAT with its owner
• Create 2 users, each with a PAT & ssh_key
• Run the following

Enable SAAS mode

export GITLAB_SIMULATE_SAAS=1
gdk restart 

• Create an ultimate Gitlab group & add in a few users
• Impersonate those users to create PATs
• Make them enterprise users (making the admin an enterprise user should be sufficient)

gdk rails console
group = Group.find(<group_id>)
user = group.users.human.last 
user.enterprise_group_id = group.id
user.save!
reload!

Requests as a top-level-group owner

curl --header "PRIVATE-TOKEN: <group_owner_pat>" "https://gdk.test:3443/api/v4/groups/<group_id/manage/personal_access_tokens" -k              

curl --header "PRIVATE-TOKEN: <group_owner_pat>" "https://gdk.test:3443/api/v4/groups/<group_id/manage/resource_access_tokens" -k

curl --header "PRIVATE-TOKEN: <group_owner_pat>" "https://gdk.test:3443/api/v4/groups/<group_id>/manage/ssh_keys" -k

MR acceptance checklist

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