[FEATURE] Add placeholder processor in Yaml import

Rework placeholder processing to allow custom processors

Resolves: #90267
Releases: master
Change-Id: If884062c09a770d5eabbc9436e1a23360290f7e2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63079


Tested-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: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Benni Mack <benni@typo3.org>