add pathServiceHandler to map endpoints to serviceId

[stevehu]

Normally, the client need to pass service_id and env_tag headers to the light-router in order to allow light-router to do the service discovery. However, if the downstream APIs have distinct endpoints and no common services like /health or /server/info are called. We can use this handler to derive the serviceId from the endpoint. The HeaderHandler can be used to set the env_tag as well so that client can just send the normal request based on the downstream APIs specification. If this is used, we need to have openapi.json specification on the router.