[2.0.0-rc1] Fix BlockSource

Problem/Motivation

As a follow-up of #3463161: [2.0.0-beta5] Do we keep BlockSource plugin?

Today we have a blacklist of non working blocks: https://git.drupalcode.org/project/ui_patterns/-/blob/2.0.x/ui_patterns....

  unset($definitions['system_messages_block']);
  unset($definitions['help_block']);
  unset($definitions['local_tasks_block']);
  unset($definitions['local_actions_block']);
  unset($definitions['system_main_block']);
  unset($definitions['page_title_block']);

Taking inspiration from similar mechanism in Layout Builder.

But some unknown blocks (custom code, or blocks from contrib modules) will not be working too.

Proposed resolution

Proposals:

• a whitelist of blocks known for working well instead of a blacklist
• OR we can add only blocks already instantiated in block layout page, for example in a hidden region
• Or we open a modal window where instanciate the block and store it somehow in a temp store and than we take it store it in our config.

Which one do we do?

API changes

No, forbidden in RC.

Data model changes

No, forbidden in RC.