Make it possible to set defaults vars for plugins based on the
AbstractPlugin.

Resolves: #17283
Releases: master
Change-Id: Ic066fcf46f299554289e30eddf4529121c088881
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/58846Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Benni Mack <benni@typo3.org>