Add optional block config to hide default blocks.

Closes #3510425

starshot_demo/config/optional/block.block.starshot_demo_page_title.yml

+uuid: 4fe0481f-dcd3-4510-833a-2479c149ab43
+langcode: en
+status: true
+dependencies:
+  theme:
+    - starshot_demo
+_core:
+  default_config_hash: 6aOgWsNTXjqrDm98TXSAjP6qd2nCijD1xw45MrnbK-Y
+id: starshot_demo_page_title
+theme: starshot_demo
+region: header_top_1
+weight: -5
+provider: null
+plugin: page_title_block
+settings:
+  id: page_title_block
+  label: 'Page title'
+  label_display: '0'
+  provider: core
+visibility: {  }