tempfork/device: add a temp fork of golang.zx2c4.com/wireguard/device

This will allow us to reuse the AllowedIPs for NAT decisions in a follow on commit.

The files `allowedips_*.go` are as-is, `peer.go` only keeps the `Peer` declaration with a
single element required for AllowedIPs.

Upstream commit https://git.zx2c4.com/wireguard-go/commit/?id=052af4a8072bbbd3bfe7edf46fe3c1b350f71f08

Updates tailscale/corp#8020

Signed-off-by: Maisem Ali <maisem@tailscale.com>
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