Synced from Eclipse Che issue

https://github.com/eclipse/che/issues/20549

(was: Check if che plug-in registry could provide the VS Code marketplace API)

Is your task related to a problem? Please describe

For offline/airgap mode, we will need a private registry to provide vsix files.
OpenVSX requires a database + elasticsearch (and oAuth is only github)

So we should check if we can propose basic API on top of existing plug-in registry to provide these offline features if people don't need a '3-tier service' for that purpose.

Describe the solution you'd like

Layer providing queries/download of VSIX files

Describe alternatives you've considered

Allow to set by preferences of the user the registry to use (like Eclipse OpenVSX instance, in-house OpenVSX, etc.)

Additional context

• [x] https://github.com/eclipse/che/issues/21598
• [x] https://github.com/eclipse/che/issues/21611
• [x] https://github.com/eclipse/che/issues/21600
• [x] https://github.com/eclipse-che/che-plugin-registry/pull/1324
• [x] https://github.com/eclipse/che/issues/21601
• [x] https://github.com/che-incubator/che-code/pull/85
• [x] https://github.com/eclipse/openvsx/pull/499
• [x] https://github.com/eclipse-che/che-operator/pull/1464
• [x] https://github.com/eclipse/che/issues/21620
• [x] https://github.com/eclipse/che/issues/21654
• [x] https://github.com/eclipse/che/issues/21662

Release Notes Text

It's now possible to run an Eclipse Che workspace using VS Code as editor in a cluster with a restricted/disconnected network (air gap). Eclipse Che is now able to run a local open-vsx registry instance within the plugin-registry pod so that VS Code editors running in workspaces can use it to manage extensions. To control what instance of open-vsx should be used in the workspaces we have introduced a new field in{{CheCluster}} CR to specify the open-vsx URL.