Internal API to get Runner info

There should be an internal API at /api/v4/internal/agents/runner/agent_info that kas can access to verify a Runner's token. This API should return the ID of the Runner (potentially more info in the future). This is similar to the existing /api/v4/internal/agents/agent{k,w}/agent_info API endpoints and should accept JWT generated by kas.

kas cannot use the existing API for token validation since it cannot authn with it using its credentials. Also, we only want a few things for now, we don't need to get everything about the runner in this API call (it will be called a lot).

Example request (identical to other agent_info endpoints):

GET /api/v4/internal/agents/runner/agent_info HTTP/1.1
Gitlab-Agent-Api-Request: <Runner token here>
Gitlab-Agent-Type: runner
Accept: application/json
Gitlab-Kas-Api-Request: <kas JWT token>

Expected response:

HTTP/1.1 200 OK
Content-Type: application/json

{
  "runner_id": 123
}