Add base xaod container

This is another attempt for: !55448 (merged) As advised by @mnowak - splitting dictionaries fixes the deadlock issue.

This MR bring the definition of the xAOD::BaseContainer. That is a container that has no actual content. It allows though to be freely decorates. It turned out that in several areas people abuse other containers for exactly that use-case, see discussion: https://groups.cern.ch/group/hn-atlas-offlineSWHelp/default.aspx?RootFolder=%2Fgroup%2Fhn%2Datlas%2DofflineSWHelp%2FLists%2FArchive%2FEMD%20class%20for%20an%20objet%20having%20only%20decorations&FolderCTID=0x012002002BD1887D3A9C0D489560C010AA1FE9F4&View={5121AF6B-9AFF-4225-8B52-9244D9184E7C}