Che-theia: Webview resources within non-theia sidecars return 404

[gattytto]

if you check the screenshot, extensions that need to serve files directly and use a sidecar return 404 errors for resources that are contained outside of the theia sidecar filesystem.

I guess this could be solved by adding workaround volume for theia in the devfile, but this is something that will be always required by certain extensions (like plantUML), so I think it would be better to add an extra property in plugin yaml volumes, like:

apiVersion: v2
publisher: gattytto
name: plantuml
version: 2.13.13
type: VS Code extension
displayName: PlantUML
title: PlantUML support
description: This extension adds support for the PlantUML Server
icon: https://github.com/gattytto/vscode-plantuml/raw/master/images/icon.png
repository: https://github.com/gattytto/vscode-plantuml
category: Language
firstPublicationDate: '2020-09-22'
spec:
  containers:
    - image: "quay.io/eclipse/che-sidecar-plantuml:2.13.13-ee2987c"
      name: vscode-plantuml
      memoryLimit: '512Mi'
      volumes:
        - mountPath: "/tmp/vscode-unpacked"
          name: "previewassets"
          **alsoMountInTheia: true**
  extensions:
    - https://github.com/gattytto/vscode-plantuml/releases/download/2.13.13/plantuml-2.13.13.vsix

[gattytto]

also changing the UML code really auto-updates the preview which is neat!

[gattytto]

in haskell:
this case we need jwt-proxy-webviews service to look for those files straight into the sidecar filesystem (and not in theia's), folders that are filled previously in the Dockerfile build stage.

In the plantUML extension case I think this is not required because the extension "fills" the content when it's initialized so it won't matter if setting the volume in extension meta.yaml erases the folder's content to have it available bot in sidecar and theia-ide container filesystem and it's just a couple files anyways.

[azatsarynnyy]

duplication of #16870