Add a way to include "extra settings"

When working on a new HLS feature, it is common to want to add some settings. Ideally, these eventually are exposed as proper settings in the extension, but when hacking on it it would be nice to have a way to toggle your new settings without having to edit the extension.

I suggest we could have something like an unstructured "extra-settings" bag, which will just get added in before sending to the server. I'm not sure whether there's an appropriate way to do this in vscode, though.