[TASK] Add FlashMessage API to Extbase AbstractController

Flow comes with an API in AbstractController to add FlashMessages.
Introducing this API to Extbase provides a convenient way to
add new FlashMessages to the queue.
Usage in your controller: $this->addFlashMessage();

Resolves: #54036
Releases: 6.2
Change-Id: I3f6b69597d7f972054c5081d1f9e20d65b598848
Reviewed-on: https://review.typo3.org/25731
Reviewed-by: Benjamin Mack
Reviewed-by: Marcin S?gol
Tested-by: Marcin S?gol
Reviewed-by: Steffen Müller
Tested-by: Steffen Müller