[!!!][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.