[TASK] Backend can read data served by envtest

[andyatmiami]

Description

k8s (and KinD) are extremely powerful platforms, but with that power comes added complexity/dependencies. A developer willing to contribute front end code to the project may be discouraged by the added overhead of requiring a k8s cluster (including KinD), particularly if they have issues setting up and/or maintaining this dependency - particularly if its not working for any variety of reasons.

We believe envtest can play a critical role in not only helping in testing, but also in enabling a more streamlined experience for frontend developers, tackling the issue above.

With the separation provided by the backend (proxy) layer, we have a nice architecture to drive this solution. We want to be able to structure the code, such that when a feature flag is set, calls from the backend to read/interact with (today) a k8s cluster could instead by serviced by envtest.

This design pattern has already been implemented by Kubeflow Model Registry.

• k8s client factory
• envtest client factory

Please note the initial requirements for this work are much simpler as compared to the current Model Registry implementation. Model Registry should only be used as a reference to help understand the basic structure of the code.
- Specifically, this issue is NOT focused on adding the support of various authentication schemes!

At a high level, the desire is to have:

• a "client factory" that encapsulates the current logic of the notebooks-v2 branch
• a "client factory" that allows reading data from envtest
• a feature flag that gives developers the ability to toggle which "mode" to run the application

Acceptance Criteria

• As a frontend developer, I should be able to run the frontend and backend process of kubeflow/notebooks locally without any k8s cluster available, and still be able to have meaningful interactions in the UI.
• At least a single namespace, with a single workspace kind, with a single workspace should be able to be retrieved in the UI

[yehudit1987]

/assign