Add 'sometimes' rule modifier

Author: VladimirBerdnik

src/Rule.php

@@ -24,6 +24,7 @@
  * @method static GenericRuleSet min($minValue) The field under validation must have a minimum value. Strings, numerics, arrays, and files are evaluated in the same fashion as the size rule.
  * @method static GenericRuleSet max($maxValue) The field under validation must be less than or equal to a maximum value. Strings, numerics, arrays, and files are evaluated in the same fashion as the size rule.
  * @method static GenericRuleSet inArray(string $anotherField) The field under validation must exist in $anotherField's values.
+ * @method static GenericRuleSet sometimes() Run validation checks against a field only if that field is present in the input array.
  * @method static GenericRuleSet requiredWith(string ...$otherFields) This field is required, if another field has value
  * @method static GenericRuleSet requiredWithAll(string ...$otherFields) The field under validation must be present and not empty only if all of the other specified fields are present.
  * @method static GenericRuleSet requiredWithout(string ...$otherFields) This field is required, if another field has no value

src/RuleSet.php

@@ -49,6 +49,7 @@ class RuleSet implements IRule
         'max',
         'between',
         'size',
+        'sometimes',
     ];
 
     /** @var array */

src/SimpleRules.php

@@ -117,4 +117,13 @@ public function size(int $value)
     {
         return $this->appendIfNotExists("size:$value");
     }
+
+    /**
+     * Run validation checks against a field only if that field is present in the input array.
+     * @return static
+     */
+    public function sometimes()
+    {
+        return $this->appendIfNotExists("sometimes");
+    }
 }