Implement Advanced Custom Fields (ACF / SCF) support

Problem/Motivation

Advanced Custom Fields (ACF by WPEngine) and Secure Custom Fields (SCF) which is now administered by an internal team, are widely used in millions of WordPress sites. We would like to offer these site admins and owners an alternative option that saves their data fields. (As we understand it both plugins currently store data in essentially the same ways but this could diverge.)

We should have a way to get ACF fields into Drupal sites whether selectively or entirely in full. This could be based on scanning the XML file, a form for manual input, or other methods.

The Craft CMS WP Import project has a large set of ACF implementations (MIT license) which could be very helpful for parsing.

We would want the ACF field import to follow the "pluggable" structure that is in the works here: #3173973: Pluggable extension, support contrib + custom WordPress, Yoast SEO. Some of our volunteer contributors has already mentioned experimenting with ACF imports!

Proposed resolution

Some type of valid import for ACF / SCF

Remaining / possible tasks

• Determine if it can be read from XML file, or defined in another way, or both.
• Possibly change YML files
• Possibly change how this module works in other ways, eg "plugin derivers"
• Alter the wizard steps to cover adding ACF / SCF fields
• Add test suite and test XML files for parsing.

User interface changes

Additional wizard step pages, or including them on the ones that exist per content type / post / page. See #3490070: Implement Custom Post Type (CPT) support regarding custom post type forms.

API changes

Depends on pluggable patch, and possibly other config changes.

Data model changes

Could change config entities or similar.