Runner controller management API (Part 2) Runner controller Token APIs

Proposal

Implement runner controller token APIs:

• List all: GET /runner_controllers/:id/tokens
• Get detail: GET /runner_controllers/:id/tokens/:token_id
• Register new token: POST /runner_controllers/:id/tokens

Revoke endpoint will be added as a follow-up: #582812, and rotate in #578798.

Notes:

• Runner controllers and tokens do not need names.
• Runner controllers and tokens need a free-form optional text description field. Make sure to set a size limit.
• Tokens must be routable so that they are compatible with Cells. See how agentk/agentw tokens are constructed.
• Tokens must use a new prefix so that we can tell what kind of token that is. Document the new prefix here https://docs.gitlab.com/security/tokens/#token-prefixes.