[Dashboard] Support proxy middleware for KubeRay history server integration

[andrewsykim]

Description

As part of the Ray History Server project, the Ray Dashboard needs to be updated to send Ray API requests to the history server. The history server component will serve the same endpoints as Ray API server but for terminated clusters.

From discussions with @alanwguo and @MengjinYan, it was suggested to add a proxy middleware in Ray Dashboard to redirect API requests to an external history server. The configuration of the proxy middleware will be via environment variables that specifies the external address to redirect requests to.

Use case

Run Ray Dashboard with history server backend to view states of terminated Ray clusters.

[andrewsykim]

@chiayi expressed interest to pick this up starting next week

[chiayi]

Yep, I will try talking a stab a it!

[Future-Outlier]

cc @CheyuWu to follow this to see if kuberay dashboard can help