Filter to extend the title block

[roborourke]

Is your feature request related to a problem? Please describe.

This is a similar request to #5821 however without the need for back compat.

There are some use-cases in our projects at Human Made where we'd like to attach some extra inputs and information to the title block that aren't entirely suited to being blocks themselves, such as alternatives for A/B testing, short title for lists vs. long title for post pages and a more cohesive & visually representative "header" section including title, date, kicker, byline, social share buttons, comment count etc...

Describe the solution you'd like

There are a few options I could see:

1. Filter to insert components before & after the title block
2. Make title block a required part of a "header" area, blocks can be registered only for this section, and a template defined for it
3. Allow a block settings sidebar context for the title block

Describe alternatives you've considered

This could be handled via a custom sidebar plugin or metaboxes (and is being done that way in the meantime).

[draganescu]

I wonder if in the context of FSE this will still be needed or, instead of a filter for title, a feature of the post block would be better.