Draft: Implement first iteration of the agents token REST API

This change set implements a first iteration of the Agents Token API at /projects/:id/agents/:agent_id/tokens.

It's based on !83270 (merged) and until that's merged this MR is subject to change, too.

e.g. there needs to be some clarification around to where to expose the API (see !83270 (merged) (comment 889367437)).

/cc @nagyv-gitlab @nmezzopera @Alexand

Please review the /agents API stuff here: !83270 (merged) !!