Reduce the number of field blocks created for entities (possibly to zero)

Problem/Motivation

Since having lots of block derivatives has various implications for performance, it would be great to only create field blocks for entities that are customised with layout builder.

I have a site where 1600 block plugins are being created for webform submission bundles, which will never require field blocks. This number will grow for every webform added to the site.

Proposed resolution

Ensure FieldBlockDeriver and ExtraFieldBlockDeriver only creates blocks it needs.

Remaining tasks

1. Get tests passing
2. Add tests for the new functionality
3. Add update path test

User interface changes

API changes

Data model changes

Release notes snippet