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/settings_test.go

@@ -34,7 +34,8 @@ func Test_Settings_lines(t *testing.T) {
 				"      |--Version: 2.5",
 				"      |--Verbosity level: 0",
 				"   |--Mullvad settings:",
-				"      |--Network protocol: udp",
+				"      |--OpenVPN selection:",
+				"         |--Protocol: udp",
 				"|--DNS:",
 				"|--Firewall: disabled ⚠️",
 				"|--System:",