add generic in/not in validation

in.go

@@ -0,0 +1,27 @@
+package validate
+
+import "errors"
+
+// In validates whether a value is found in a slice of allowed values.
+func In[T comparable](allow []T) func(T) error {
+	return func(value T) error {
+		for _, cmp := range allow {
+			if cmp == value {
+				return nil
+			}
+		}
+		return errors.New("Not an allowed value")
+	}
+}
+
+// NotIn validates whether a value is not found in a slice of disallowed values.
+func NotIn[T comparable](allow []T) func(T) error {
+	return func(value T) error {
+		for _, cmp := range allow {
+			if cmp == value {
+				return errors.New("Not an allowed value")
+			}
+		}
+		return nil
+	}
+}