Implement a scope for VaadinSession

[vaadin-bot]

Originally by @Legioth

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As discussed in #19975, there are some cases where there isn't an 1-to-1 mapping between @SessionScoped and the lifecycle of VaadinSession. For his reason, it would be useful to have a separate scope for the lifecycle of VaadinSession, i.e. @VaadinSessionScoped.

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Imported from https://dev.vaadin.com/ issue #19979

[mrts]

@tsuoanttila this seems to be fixed now with the introduction of @VaadinSessionScoped?

I want to specially thank you all for making this happen - @VaadinSessionScoped, CDINavigator, @PushStateNavigation and Transport.WEBSOCKET_XHR for @Push finally enable building real time Java EE web apps with clean navigation and secure session handling in a really clean and neat way :).