chore(validation): move functions from constants

- Move validation functions from `internal/constants` to `internal/configuration/settings/validation`
- Concatenate all OpenVPN constants in `internal/constants/openvpn.go`

internal/constants/privateinternetaccess.go

@@ -0,0 +1,7 @@
+package constants
+
+const (
+	PIAEncryptionPresetNone   = "none"
+	PIAEncryptionPresetNormal = "normal"
+	PIAEncryptionPresetStrong = "strong"
+)