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/constants/vpn/protocol.go

@@ -1,6 +1,7 @@
 package vpn
 
 const (
+	AmneziaWg = "amneziawg"
 	OpenVPN   = "openvpn"
 	Wireguard = "wireguard"
 )