Configure open-vsx as the extension marketplace for our server

Currently, we configure the https://open-vsx.org/ as the marketplace on the client (WebIDE) https://gitlab.com/gitlab-org/gitlab-web-ide/-/blob/8f9547afdb4ce7f673a52e6543427fdfa1bf65bd/packages/vscode-bootstrap/src/start.ts#L45-50

We have to do the same thing for the server, which will involve changing the product.json template: https://gitlab.com/gitlab-org/gitlab-web-ide-vscode-fork/-/blob/22c16c4091f696a2056fe020c9ba392775e02389/product.json#L2

We should add the following setting:

      "extensionsGallery": {
        "serviceUrl": "https://open-vsx.org/vscode/gallery",
        "itemUrl": "https://open-vsx.org/vscode/item",
        "resourceUrlTemplate": "https://open-vsx.org/vscode/asset/{publisher}/{name}/{version}/Microsoft.VisualStudio.Code.WebResources/{path}"
      }

Until we add this setting to the build, we need to add this setting to the product.json in the unpacked server distribution: gitlab-web-ide!64 (diffs)