\Drupal\block_content\Plugin\Derivative\BlockContent::getDerivativeDefinitions could use an aggregate query instead of loadMultiple

Problem/Motivation

Similar issue as #2940755: block_content block derivatives do not scale to thousands of block_content entities but a different approach to a fix

Steps to reproduce

Have 1000s of reusable block content entities.
See your site die with OOM on plugin definition rebuild.

Proposed resolution

Use an aggregate query instead of ::loadMultiple

Merge request link

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet