Refactor the IA rendering for OG

[vkama]

Refactor the whole implementation of the IA Markup Generation for the OG Ingestion, to be more in line with the way it was done in the IA->AMP.

This would include, but not limited to:

• [ ] Creating a class for the whole code
• [ ] Moving it to a separate .php file
• [ ] Moving the ::Init() code to the main plugin php to make it more explicit
• [ ] Renaming ia_markup() method to render_ia()
• [ ] Adding filters and hooks

See #687 for a similar issue