Problem/Motivation
There's an issue to support adding third party settings to sections: #2942661: Sections should have third-party settings, but it would be great to also support third party settings for individual components (blocks) within a section.
For example, modules like Block Class, Field Formatter Class, Panopoly or Fences Block use third party settings on block config entities to allow the user to add arbitrary CSS classes to a block. It seems that the only way to do this with a Layout Builder block is to modify the settings form for the individual block plugin you want to modify and store the configuration along with the other configuration for that block.
Proposed resolution
SectionComponent should implement ThirdPartySettingsInterface
Remaining tasks
N/A
User interface changes
N/A
API changes
Because there was an existing similar approach ($additional)), deprecations are added with full BC
Data model changes
Data model addition
Release notes snippet
N/A