Publish embedded VSCode extensions at build time

At this time when the embedded plugin registry is activated (openVSXURL: ''), all VS Code extensions described in openvsx-sync.json are publishing into the local openvsx registry during the plugin-registry pod initialization and it takes some time (> 2 minutes).

Need to investigate and implement if it's possible the way to publish extensions into the registry during the image's build process. It could make pod's initialization much faster when the embedded plugin registry is activated.