Secure access to the debugger only to Metabase employees

[thebiglabasky]

We should secure access to the debugger to avoid leaking information about bug reports or adding exposed surface.

In terms of solution the simplest way would likely be to implement something like next-auth as a first step.
An alternative might be to make the debugger require our Tailscale VPN, but I assume this would be more involved in how we deploy it.

Whatever is the simplest first wins imo.