Api endpoint list all group pats

What does this MR do and why?

#510346 (closed)

Introduced an endpoint for group owners to list PATs for the users in a group. This feature is currently behind a feature flag

Screenshots or screen recordings

Curl request showing forbidden response for a user who is not group owner

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Perform following steps in the console

1. Find the group in your local setup by id/group_name and non-bot user belonging to the group

 group = Group.find(<group_id)
 user = group.user.human.first

2. Create an enterprise user for a group by setting enterprise_group_id field of the user to group's id -

user.enterprise_group_id = group.id

3. Create Personal access token for the user using following api

https://docs.gitlab.com/ee/api/user_keys.html#add-an-ssh-key-for-a-user

4. Try using the api endpoint introduced in the MR with pat created for for the group owner with api access

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

Tokens created for user in step 3 should be listed.

Try using filters like created_before, created_after, expires_before etc and the api call should return appropriate response.