Add support for deno debugger

It would be really useful to be able to start the deno debugger in the deno-relay container with the appropriate port exposed so that a compliant inspector can attach to it.

I'd be happy to have a go at implementing this if you agree it's worthwhile.

It would involve the following:
- Add an argument to supabase functions server to enable the debugger
- Update the code to expose the relevant port
- Some investigation and implementation to make sure that things like breakpoints work sensibly.