Configurable download location for vscode-server

I work in an environment where an end user's ability to download software from the internet on to their desktop is restricted. We'd like enable the use of the Remote SSH extension, but the current installation mechanism for vscode-server doesn't work for us. The target hosts lack internet access to pull down the tarball, and we cannot to use the capability exposed by the remote.SSH.allowLocalServerDownload flag as this would result in a direct download to the user's desktop machine.

I'd like to be able to configure a download location for vscode-server, allowing us to serve the tarball from inside our network. This would require the extension to expose a configuration property through which the base download url could be modified, defaulting to https://update.code.visualstudio.com.

A workable alternative for us would be an offline mode for the Remote SSH installation, as described by https://github.com/microsoft/vscode-remote-release/issues/1399.

Start by tracing the Remote SSH server installation path and the existing remote.SSH.allowLocalServerDownload configuration. Compare the requested configurable base URL with the offline-mode discussion in issue 1399; done means the server tarball can be served from an internal location while retaining https://update.code.visualstudio.com as the default.