use the embed/embed library to retrieve website card data

[goto-bus-stop]

This PR replaces the opengraph.io API usage with a PHP library that can run on your own wordpress site without any API key.

I used embed/embed 3.x because the web host where I'm using this doesn't have PHP 7.4 yet, and embed/embed v4 and up require PHP 7.4 :grimacing:

I configured composer to put dependencies in a deps/ folder, so it doesn't conflict with the existing vendor/ folder.

Closes https://github.com/maximebj/agb.plugin/issues/60

[maximebj]

Seems great, and cleaner ! I'll try to approve this and merge soon but I have a lot to do in the next weeks. I'll try to fix my bugs and merge contributions before my hollidays at the end of the month

[goto-bus-stop]

thanks! there's no pressure from my side, so take your time :)