FR: Use of post-quantum cryptography (Rosenpass, PSK)

### What are you trying to do?

Hello,

it would be a nice feature to use post-quantum cryptography.

### How should we solve this?

WireGuard appears to support post-quantum secure cryptography when using a PSK. In addition, there is the [Rosenpass project](https://rosenpass.eu/), which supports secure negotiation of a PSK.

### What is the impact of not solving this?

At some point in the future, tailscale may no longer be very secure.

### Anything else?

_No response_

The issue names no files, tests, or entry points. Start by reading the linked WireGuard PSK and Rosenpass material, then locate the relevant cryptography and networking areas in Tailscale. Done would require an agreed design and verified post-quantum support, including appropriate security tests.

Written by the indexing model from the issue text.