Add an ability to turn multiplexer on and off in general settings.

It would be easier to debug if any issues are related to websocket in general or specific to multiplexer.

Describe the solution you'd like

Currently people need to use an environment variable, but this means only people who build headlamp can try this feature.

In the settings area it should also mention "Experimental feature", or maybe even a whole section of experimental features, and this setting could go under that "Experimental feature" heading.

What users will benefit from this feature?

Everyone who needs a "web socket multiplexer". That is people with multiple clusters, or who are looking at many resources at once.

It should still respect the environment variable: REACT_APP_ENABLE_WEBSOCKET_MULTIPLEXER

If that is true then it should be enabled, if false disabled.

Should the user setting over ride this? Yes. If the user has not changed the setting at all, then just use the environment variable.

Additional context

See previous multiplexer PRs

https://github.com/kubernetes-sigs/headlamp/pulls?q=is%3Apr+multiplexer+is%3Aclosed

See frontend/src/lib/k8s/api/v2/useKubeObjectList.ts

Currently you need to set this when running or building frontend.

REACT_APP_ENABLE_WEBSOCKET_MULTIPLEXER=true npm run frontend:build
REACT_APP_ENABLE_WEBSOCKET_MULTIPLEXER=true npm run frontend:start