chore(config): define Source interface locally where needed

internal/cli/interfaces.go

@@ -0,0 +1,9 @@
+package cli
+
+import "github.com/qdm12/gluetun/internal/configuration/settings"
+
+type Source interface {
+	Read() (settings settings.Settings, err error)
+	ReadHealth() (health settings.Health, err error)
+	String() string
+}