Tighten types for Hydrator and its subclasses to address issue caught by Psalm.

The issue caught by Psalm:

Argument 2 of Mailgun\Hydrator\ModelHydrator::hydrate has the more specific type 'class-string', expecting 'string' as defined by Mailgun\Hydrator\Hydrator::hydrate

src/Hydrator/ArrayHydrator.php

@@ -22,6 +22,8 @@ use Psr\Http\Message\ResponseInterface;
 final class ArrayHydrator implements Hydrator
 {
     /**
+     * @phpstan-param class-string $class
+     *
      * @return array
      */
     public function hydrate(ResponseInterface $response, string $class)

src/Hydrator/Hydrator.php

@@ -20,6 +20,8 @@ use Psr\Http\Message\ResponseInterface;
 interface Hydrator
 {
     /**
+     * @phpstan-param class-string $class
+     *
      * @throws HydrationException
      */
     public function hydrate(ResponseInterface $response, string $class);

src/Hydrator/NoopHydrator.php

@@ -21,6 +21,8 @@ use Psr\Http\Message\ResponseInterface;
 final class NoopHydrator implements Hydrator
 {
     /**
+     * @phpstan-param class-string $class
+     *
      * @throws \LogicException
      */
     public function hydrate(ResponseInterface $response, string $class)