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/configuration/settings/serverselection.go

@@ -87,7 +87,7 @@ func (ss *ServerSelection) validate(vpnServiceProvider string,
 	filterChoicesGetter FilterChoicesGetter, warner Warner,
 ) (err error) {
 	switch ss.VPN {
-	case vpn.OpenVPN, vpn.Wireguard:
+	case vpn.AmneziaWg, vpn.OpenVPN, vpn.Wireguard:
 	default:
 		return fmt.Errorf("%w: %s", ErrVPNTypeNotValid, ss.VPN)
 	}