Сonstruct structured agentk tokens

As per discussion in &18384 (comment 2633450371), we need to construct structured agentk tokens so that Cells HTTP router can send the request to the correct cell.

Should be similar to agentw tokens, see https://gitlab.com/gitlab-org/gitlab/blob/79b7d787e147cc2397cfbc997f04192ea9695c8f/ee/app/models/remote_development/workspace_token.rb#L9.