chore!(amneziawg): refactor to be separate from wireguard

- amneziawg is now a VPN protocol and no longer a Wireguard implementation
- Use it with VPN_TYPE=amneziawg
- document AMNEZIAWG_* options in Dockerfile
- document amneziawg support in readme
- separate amneziawg settings and code from wireguard
- re-use code from wireguard whenever possible

internal/amneziawg/helpers_test.go

@@ -0,0 +1,5 @@
+package amneziawg
+
+func ptrTo[T any](v T) *T {
+	return &v
+}