Maint: configuration Openvpn selection structure

- Move network protocol from ServerSelection to OpenVPNSelection child
- Move PIA encryption preset from ServerSelection to OpenVPNSelection child
- Move custom port from ServerSelection to OpenVPNSelection child

internal/configuration/purevpn.go

@@ -23,17 +23,14 @@ func (settings *Provider) purevpnLines() (lines []string) {
 		lines = append(lines, lastIndent+"Hostnames: "+commaJoin(settings.ServerSelection.Hostnames))
 	}
 
+	lines = append(lines, settings.ServerSelection.OpenVPN.lines()...)
+
 	return lines
 }
 
 func (settings *Provider) readPurevpn(r reader) (err error) {
 	settings.Name = constants.Purevpn
 
-	settings.ServerSelection.TCP, err = readProtocol(r.env)
-	if err != nil {
-		return err
-	}
-
 	settings.ServerSelection.TargetIP, err = readTargetIP(r.env)
 	if err != nil {
 		return err
@@ -59,5 +56,5 @@ func (settings *Provider) readPurevpn(r reader) (err error) {
 		return fmt.Errorf("environment variable SERVER_HOSTNAME: %w", err)
 	}
 
-	return nil
+	return settings.ServerSelection.OpenVPN.readProtocolOnly(r.env)
 }