[TASK] Introduce type declarations in extbase's Session class

Releases: main
Resolves: #100068
Change-Id: Ibc06f9c6b227f956290e02cc9bbd44587bac6f78
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77851


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Benni Mack <benni@typo3.org>