Che Theia Web View doesn't work properly when a plugin is running in its own sidecar

[azatsarynnyy]

Describe the bug

Che Theia Web View doesn't work properly when a plugin is running in its own sidecar.

Steps to reproduce

1. Start a Workspace with the following plugin descriptor:

apiVersion: v2
publisher: redhat
name: vscode-didact
version: 0.1.14
type: VS Code extension
displayName: YAML
title: Didact Tutorial Tools by Red Hat
description: Provides interactive tutorial support to Visual Studio Code, via didact links calling VS Code commands, Markdown, AsciiDoc, and the VS Code webview.
icon: https://www.eclipse.org/che/images/logo-eclipseche.svg
repository: https://github.com/redhat-developer/vscode-didact
category: Other
firstPublicationDate: "2020-05-06"
spec:
  containers:
    - image: "quay.io/eclipse/che-sidecar-node:10-0cb5d78"
      name: vscode-didact
      memoryLimit: "256Mi"
  extensions:
    - https://github.com/redhat-developer/vscode-didact/releases/download/0.1.14/vscode-didact-0.1.14.vsix

2. Go to DIDACT TUTORIALS view and run Didact Demo
   

3. The page is opened in Theia Web View has no styles applied:
   

Also, the links that calling the commands aren't working.

Expected behavior

The page is opened in Theia Web View should have the styles applied:

The links that calling the commands should work.

Additional context

From the browser's dev console:

main.js:469 GET https://routegp3bizjr-azatsary-che.8a09.starter-us-east-2.openshiftapps.com/webview/theia-resource/file/tmp/vscode-unpacked/redhat.vscode-didact.0.1.14.mnjqnseceg.vscode-didact-0.1.14.vsix/extension/media/webviewslim.css net::ERR_ABORTED 404
(anonymous) @ main.js:469
setTimeout (async)
(anonymous) @ main.js:466
main.js:469 GET https://routegp3bizjr-azatsary-che.8a09.starter-us-east-2.openshiftapps.com/webview/theia-resource/file/tmp/vscode-unpacked/redhat.vscode-didact.0.1.14.mnjqnseceg.vscode-didact-0.1.14.vsix/extension/media/main.js net::ERR_ABORTED 404

Sub-tasks

• Implement simple file system provider to fetch files from containerized plugin Implement simple file system provider to fetch files from containerized plugin #18405

[azatsarynnyy]

I think retrieving the plugin resources for WebView can be done the similar way as we've fixed fetching the icons for the contributed Views #13750 in eclipse-che/che-theia#430

[ericwill]

It would be great if this could make it into 7.22, as there are a number of extensions that are facing this issue.

[gattytto]

sometimes the files are downloaded/provided by the extension at runtime, but in other scenarios files are already there from the Dockerfile build process..