Short description of what this feature will allow to do:
Allow attaching actions to specific fields to be rendered in the detail view.

Example of how to use this feature
If you have an a entity with a lot of custom actions they start cluttering the menu REALLY fast making it hard to navigate them.
As such it would be great if you could attach actions to specific fields, so they'd be rendered below the field value on the details page.

This would allow the developer to break up and place the actions anywhere in the view (real field or virtual field) and would allow for greatly improved UI building since you don't have to cram every action into the menu anymore.

As for how to attach them, I'd say there's two possibilities here:

1. Expand the Actions:add method with an optional 3rd parameter for the field:

$actions->add(Crud::PAGE_DETAIL, $myAction, 'foo');

2. Define the actions by names when defining the fields:

TextField::new('foo')->setActions(['fooAction','barAction']);

After an action is attached to a field in such a way it should move out of the header and into the detail view.