Add endpoint to web server showing client details, and config

[anderseknert]

Regal's language server already starts a web server at some random available port for pprof / statsviz data. It would be great if we could add another very simple endpoint where one could go to see the payload sent by the client to initialize Regal. This would help a lot with troubleshooting and reproducing issues that we suspect are related to what the client sends. We'd have to ask the user to share this of course, but that is way easier than having them run with some debug configuration, and try to find relevant data from a million lines logged.