Implement Runner Controller API

The goal of this issue is to implement the runner controller and its token REST API endpoints.

There is no documentation available yet, but will be when Documentation for Runner controller and token API (gitlab-org/gitlab#581275) gets done.

The implementation of the endpoints which can be used as a reference to implement the API in client-go here are at:

• https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/api/ci/runner_controllers.rb
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/api/ci/runner_controller_tokens.rb

Consider the following:

• we don't have docs yet, mention that in the API endpoint godoc accordingly with a reference to the aforementioned issue
• implement unit tests in the existing style
• mention in the godoc that it's an admin only endpoint