In Ray 2.0, we want to achieve api server HA.
Originally serve endpoints are in head node.
This pr moves serve endpoints to dashboard agents, so they will be HA due to multiple replica of dashboard agent.
The Ray Dashboard starts Serve in the `"_ray_internal_dashboard"` namespace. However, Serve by default starts in the `"serve"` namespace. This causes surprising behavior when working with the Serve CLI and REST API.
This change make the Ray Dashboard start Serve in the `"serve"` namespace, allowing the REST API to work intuitively with the Python API.
The concept of a Serve Application, a data structure containing all information needed to deploy Serve on a Ray cluster, has surfaced during recent design discussions. This change introduces a formal Application data structure and refactors existing code to use it.
This change adds the GET, PUT, and DELETE commands for Serve’s REST API. The dashboard receives these commands and issues corresponding requests to the Serve controller.
