Add an event handler for accordion opening/closing at users' direct request.

A function prop that can be used to perform actions like load data or send focus to the contents of the accordion.

Use-case: sending focus to the contents of an accordion when its's opened by a user.

• it cannot be based on open/closed state alone - the user has to have clicked the header for it to trigger.

This only applies to the header of the accordion, not the collapse/expand all link.