Add basic EventTag writing new config

Merged Tadej Novak requested to merge tadej/athena:eventtag into master

As I did not see any config for it, I added new-style configuration for writing out EventTag. I simplified it a bit for now and we can add extra flags/arguments later as needed. Note that I did not introduce a special flag for disabling it as this will be mainly a special case (or a temporary workaround until all code is migrated to xAOD::EventInfo).

/cc @tsulaia @gemmeren

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