media_sideload_image should have option to return ID

[kraftbj]

media_sideload_image can either return the URL of the sideloaded image or a full HTML tag. It should have an option to return the attachment ID as well.

One use case is to sideload an image, then set it as a the post_thumbnail.

This is currently possible via media_handle_sideload but requires duplication of the work already being done in media_sideload_image.

[kraftbj]

Adds new id return.

[dd32]

Although a bit of a mess, see #19629