Validate composer.json on extension upload

Background

TYPO3 moves more and more into "composer first". This feature is describing it best.
In TER we still validate only the ext_emconf.php file and do only check the existence of a composer.json and some of its content.

ACs

• Add a check for a valid composer.json on extension upload (similar to this minimal composer.json)
  • name is set
  • description is set
  • license is set
  • type is typo3-cms-extension
  • extras/typo3-cms/extension-key is set
  • requires typo3/cms-core
• Return a warning if composer.json is invalid or not existing on extension upload until date XX.XX.2022
• Return an error and deny extension upload after date XX.XX.2022