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/constructor.go

@@ -0,0 +1,22 @@
+package amneziawg
+
+type Amneziawg struct {
+	logger   Logger
+	settings Settings
+	netlink  NetLinker
+}
+
+func New(settings Settings, netlink NetLinker,
+	logger Logger,
+) (a *Amneziawg, err error) {
+	settings.SetDefaults()
+	if err := settings.Check(); err != nil {
+		return nil, err
+	}
+
+	return &Amneziawg{
+		logger:   logger,
+		settings: settings,
+		netlink:  netlink,
+	}, nil
+}