Media: Import images Alt text IPTC metadata.

This changeset introduce a new `wp_get_image_alttext()` function that extracts Image Alt text metadata from image IPTC metadata.

Props jhmonroe, rishabhwpn, adamsilverstein, sajjad67, ozgursar, joedolson, audrasjb, huzaifaalmesbah, sabernhardt, valentingrenier, louischan, penelopeadrian, mathiaspeguet.
Fixes #63895.



git-svn-id: https://develop.svn.wordpress.org/trunk@61841 602fd350-edb4-49c9-b593-d223f7449a82