ESM Auth Server

Authorization for Ubuntu Pro related Debian repositories

This Go server coordinates with nginx for handling ESM authorization, using the nginx auth_request system. The server returns a 200 response with an empty body if the access is granted, a 401 response with an empty body otherwise. When a request arrives, we extract the resource token from the auth header and use the /v1/resources/esm?token=... endpoint on the contract service to validate that the resource token can perform the request. The resource token will be provided in the request header, the entitlement will be returned in the response body.