Filter for wp_safe_remote_get in download_url

[realloc]

I had to deal with images on a server that has basic authentication active. There is no filter in download_url that would give me the chance to add the header authentication that I needed in this case.

[donmhico]

I added a new patch for this issue.

Here are some key points about the new patch I attached 40153.2.diff.

1. Added docs regarding the new filter named download_url_remote_get_args which was introduced.
2. Unlike the first patch, I think it's best to not allow the new filter to override $args['timeout'], $args['stream'], and $args['filename'].

$args['timeout'] can be debatable. But I think it's enough that download_url() itself accepts the $timeout params.
$args['filename'] and $args['stream'] unless there's a good reason, these 2 should also be left untouched IMHO to prevent any unexpected issues to arise with how download_url() currently works.