Cannot resolve /hostedPlugin/ Resources from Sidecar Containers

[tsmaeder]

Describe the bug

When testing the Java test runner plugin in Che, I noticed that some icons symbolizing "run", "pass", etc. could not be shown (#14871 (comment)). It turns out that the request for such icons (https://routeij17ga9z-che.apps-crc.testing/hostedPlugin/vscode-java-test/resources/media/dark/pass.svg) fails with code 404.
Such URI's are usually resolved here, but since the icon is not present in the Theia container, but only in the sidecar container running the plugins, the request fails with "not found".

Che version

che-theia master as of today.

Steps to reproduce

1. Add the java test runner plugin to a java-maven workspace, by using this plugin URI: https://raw.githubusercontent.com/tsmaeder/che-plugin-registry/17025_update_java_extensions/v3/plugins/redhat/java11/0.63.0/meta.yaml
2. From the test explorer, run a test via the context menu
3. Observe: there are no icons visible and the log shows a 404 for uri's like the one above.

Expected behavior

The icon needs to show up.

Browser Log:

CellMeasurer.js:102 GET https://routeij17ga9z-che.apps-crc.testing/hostedPlugin/vscode-java-test/resources/media/dark/pass.svg 404 (Not Found)

[tsmaeder]

This works now agains che-theia:next