Feature: #93063 - FlashMessages are stored in session as JsonSerializable

See Issue #93063

Description

FlashMessages which are used to show information across backend modules and frontend plugins / forms, are mostly stored in the session data of a user session.

They are now stored as json_encoded data, using the already existing JsonSerializable functionality of AbstractMessage.

Impact

This way, the FlashMessage objects are only built when they are needed and not on every PHP call the user session is started, making e.g. AJAX calls a tiny bit faster.