[FEATURE] Add Meta Tag API

TYPO3 now allows to set, get and unset meta tags with an clear and
concise API. This API allows it to set any meta tag via an extension
while having the possibility to get the current content of a given meta
tag.
Unsetting meta tags allows to remove unwanted tags from 3rd party code.

Resolves: #81464
Resolves: #22992
Resolves: #52471

Releases: master
Change-Id: I31c2c29e29242ad9c2fadf2b625ae04288cc3bb5
Reviewed-on: https://review.typo3.org/53110


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>