[12.x] HTTP Client: add mergeUrlParameters() to combine URL parameters without overwriting (#57282)

* Implement mergeUrlParameters method

Add method to merge URL parameters into existing map.

* Update PendingRequest.php

---------

Co-authored-by: Taylor Otwell <[email protected]>

Author: leek

src/Illuminate/Http/Client/PendingRequest.php

@@ -528,7 +528,7 @@ public function withUserAgent($userAgent)
     public function withUrlParameters(array $parameters = [])
     {
         return tap($this, function () use ($parameters) {
-            $this->urlParameters = $parameters;
+            $this->urlParameters = array_merge($this->urlParameters, $parameters);
         });
     }