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.