Fix wrong type received by "KeySet" rule

I've defined the type that `KeySet` would receive as `Key` because, in
fact, it should only accept `Key` rules.

However, the `Validator::__callStatic()` method (called when you run
`v::key(...)`) will create an instance of itself, not the instance of
`Key`, and that's because you might want to add more rules to the chain.
Knowing that, we did some workaround in which `KeySet` will check if the
given rules are either a `Key` or a `Validator` with only one Key rule.

This commit will replace the type from `Key` to `Validatable`, and let
the implementation deal with the wrong type.

library/ChainedValidator.php

@@ -10,7 +10,6 @@
 namespace Respect\Validation;
 
 use finfo;
-use Respect\Validation\Rules\Key;
 
 interface ChainedValidator extends Validatable
 {
@@ -199,7 +198,7 @@ public function keyNested(
         bool $mandatory = true
     ): ChainedValidator;
 
-    public function keySet(Key ...$rule): ChainedValidator;
+    public function keySet(Validatable ...$rule): ChainedValidator;
 
     public function keyValue(string $comparedKey, string $ruleName, string $baseKey): ChainedValidator;