add callback field

[Florian-Merle]

The Callback column aims to offer almost as much flexibility as the Twig column, but without requiring the creation of a template.
You simply need to specify a callback, which allows you to transform the 'data' variable on the fly.

[diimpp]

To match existing documentation, please add YAML example as well.

[Florian-Merle]

I did not include documentation for the YAML format because I believe it's impossible to define a callback field in this format.

[diimpp]

Feature is not complete then and cannot be merged until YAML support will be dropped.

Here is example on how callback can be implemented with non-php config format.

https://symfony.com/doc/current/reference/constraints/Callback.html#external-callbacks-and-closures

[loic425]

I think we can do it. The repository can be a callback in YAML.

app_book_by_english_authors:
    driver:
        name: doctrine/orm
        options:
            class: App\Entity\Book
            repository:
                method: [expr:service('app.english_books_query_builder'), create]