[!!!][TASK] Refactor generation of extensions.xml.gz

This change re-implements the generation of the XML file for TYPO3 installations by moving the relevant code from EXT:ter to ter_fe2.

CombinedExtensionRepository -> does the collection of available extensions ExtensionIndexService -> handles the XML generation and writing to files

This way, all functionality can be separated and exchanged if necessary.

A CLI command ter:createExtensionIndexXml is added which also shows some more useful output, however the logic is still in the "UpdateCurrentVersionListTask" class in EXT:ter.

The methods:

• tx_ter_helper->requestUpdateOfExtensionIndexFile
• tx_ter_helper->writeExtensionIndexfile

are removed, and thus, all of this logic is migrated to Doctrine DBAL.

Root composer.json is adapted so the necessary PHP extensions are available in your IDE.