The block connections concept received a major overhaul. Now, blocks can be connected to custom fields and future data sources. This works in a 3-step process:
- fetching connected block attributes
- collecting values from connected sources,
- and finally, updating the block's HTML with these values.

A new helper function called 'gutenberg_render_block_with_connection_value' is introduced which replaces either the whole HTML tag or a specific attribute with the connection value, based on the source and configuration parameters.

Also, adjusted the behavior in block-editor's custom fields when an attribute's value is empty. Instead of setting it as undefined, it is now set as an empty string, ensuring smoother handling.