put largefile field extend to service provider

[xiaohuilam]

https://github.com/laravel-admin-extensions/large-file-upload/blob/861fb27c74084456340fe0fb2e466eb8af761499/README.md#L21-L24

I suggest to put it into service provider's boot

    public function boot()
        Admin::booting(function () {
             \Encore\Admin\Form::extend('largefile', \Encore\LargeFileUpload\LargeFileField::class); 
        });
    }