Enable installation of VSCode extensions from openvsx.org via the internal OpenVSX registry

[svor]

Is your task related to a problem? Please describe

This PR enables the setup of a private OpenVSX registry: eclipse/openvsx#1011

Watch the screencast to see how it works:
Deploy_on_OpenShift.webm

When configure Eclipse Che to use the custom OpenVSX registry, it's possible to see all extensions that are published into the internal registry and install them from the editor:

In case the custom OpenVSX registry is configured to use https://open-vsx.org as an upstream registry (it's a part of application.yml server configuration file):

ovsx:
  databasesearch:
    enabled: false
  upstream:
    url: https://open-vsx.org

We can see all extensions (internal and from openvsx.org) in the custom OpenVSX UI page:

But in Che we can NOT see them in the Extensions View:

Logs from openvsx-server pod:

2024-10-08T12:17:55.982Z ERROR [openvsx-server,f35c64024fd055f884121939131e09e2,66374e3386a84ad4] 1 --- [openvsx-server] [tp1853443923-20] [f35c64024fd055f884121939131e09e2-66374e3386a84ad4] o.e.o.adapter.UpstreamVSCodeService      : upstream: POST: https://open-vsx.org/vscode/gallery/extensionquery

org.springframework.web.client.ResourceAccessException: I/O error on POST request for "https://open-vsx.org/vscode/gallery/extensionquery": open-vsx.org:443 failed to respond
	at org.springframework.web.client.RestTemplate.createResourceAccessException(RestTemplate.java:915) ~[spring-web-6.1.5.jar:6.1.5]
	at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:895) ~[spring-web-6.1.5.jar:6.1.5]
	at org.springframework.web.client.RestTemplate.exchange(RestTemplate.java:730) ~[spring-web-6.1.5.jar:6.1.5]
	at org.eclipse.openvsx.adapter.UpstreamVSCodeService.extensionQuery(UpstreamVSCodeService.java:66) ~[classes/:na]
	at org.eclipse.openvsx.mirror.MirrorExtensionQueryRequestHandler.getResult(MirrorExtensionQueryRequestHandler.java:48) ~[classes/:na]
	at org.eclipse.openvsx.adapter.VSCodeAPI.extensionQuery(VSCodeAPI.java:68) ~[classes/:na]

Describe the solution you'd like

Extensions view should show extensions from upstream registry as well