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

[leek]

This PR introduces mergeUrlParameters(array $parameters = []) to Illuminate\Http\Client\PendingRequest.

Today, withUrlParameters() replaces the entire parameter map, which makes it hard to define global parameters (e.g., in a macro) and then add/override endpoint-specific parameters later. The new method merges into the existing map, letting later calls override only the keys they provide.

Backwards compatibility

• Fully BC: existing withUrlParameters() behavior is unchanged.
• No impact on public signatures other than the additive new method.

Usage example

Http::macro('service', function () {
    return Http::baseUrl('https://api.example.com')
        ->withUrlParameters(['version' => 'v1', 'tenant' => 'acme']);
});

$response = Http::service()
    ->mergeUrlParameters(['tenant' => 'beta'])
    ->get('/{version}/tenants/{tenant}/users');

[taylorotwell]

I kinda think they should just always merge.

[guenbakku]

It looks like that the MR title doesn't quite match the final code. There is no new method mergeUrlParameters; instead, the behavior of withUrlParameters was updated to merge (instead of replace).