Add support for headless websites

Problem/Motivation

If we want to use this module with headless Drupal websites, we can't rely on field formatters.

Proposed resolution

Add a URI base field to the file entity type containing the base64 formatted placeholder image, so it can be consumed by e.g. JSON:API.

User interface changes

None.

API changes

A image_blurry_placeholder base field is added to the file entity type.

Data model changes

None.