Enhance QueryBuilder with generics support for better type inference

[alexkart]

This pull request includes several changes to the QueryBuilder class in src/QueryBuilder.php to improve type safety and code clarity by adding template annotations and type hints. The most important changes include adding a template annotation to the class, updating the return type annotations for methods, and ensuring proper type casting.

When trying to specify the correct type in the following code:

/**
     * @return LengthAwarePaginator<int, Client>
     */
    public function queryClients(Request $request): LengthAwarePaginator
    {
        /** @var QueryBuilder<Client> $queryBuilder */
        $queryBuilder = QueryBuilder::for(Client::class, $request);
...

Static analysis (PHPStan) generates the following error:

Method Modules\Client\Repositories\ClientRepository::queryClients() should return Illuminate\Pagination\LengthAwarePaginator<int, Modules\Client\Models\Client> but returns  
         Illuminate\Pagination\LengthAwarePaginator<int, Illuminate\Database\Eloquent\Model>.                                                                                         
         🪪  return.type

It happens because Illuminate\Database\Eloquent\Builder is a generic class:

/**
 * @template TModel of \Illuminate\Database\Eloquent\Model
 */
class Builder implements BuilderContract

but \Spatie\QueryBuilder\QueryBuilder that specifies this class as a @mixin is not generic, so there is no way to specify a concrete type for the TModel.
This PR adds the ability to do this.

[freekmurze]

Thanks!