Implement GET API for GATs and PATs

Closes #355893 (closed)

Again, this change is motivated by the Terraform GitLab Provider which currently uses the list API to obtain the details of a single Group or Project Access Token.

This change set implements the GET API for a single Group and Project Access Token.

It also introduces a schema fixture to verify the response for the list responses for Group and Project Access Tokens.

It also makes sure that the response contains the proper pagination headers.

I've recently learned during another Review for one of my MRs that a leading The for field descriptions is not necessary (see !82467 (merged) (comment 869500543)).

Therefore, I have omitted it for the GET APIs and this change now aligns the documentation for the Group and Project Access Tokens.

/cc @nagyv-gitlab @nmezzopera

Edited by Hannah Sutor