extraData for ExpandRowColumn should allow for a closure so we can pass model information

[adamwinn]

Prerequisites

• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• The issue still exists against the latest master branch of yii2-grid.
• This is not an usage question. I confirm having gone through and read the documentation and demos.
• This is not a general programming / coding question. (Those should be directed to the webtips Q & A forum).
• I have attempted to find the simplest possible steps to reproduce the issue.
• I have included a failing test as a pull request (Optional).

Steps to reproduce the issue

extraData only allows for a simple array, but it would be nice to be able to access model information with it.

Something like this:

$col['extraData'] = function ($model, $key, $index, $column) {
            return [
                    'event_id' => $model['event_id']
            ];
        };