Store pile-up flag in digitization metadata

Tadej Novakrequested to merge
tadej/athena:digi-meta-pileup into 22.0

Store pile-up flag in digitization metadata to be able to differentiate RDOs with and without pile-up.

The first client is xAODTruthCnvAlg. Fixes ATLASRECTS-7211.

/cc @mhodgkin @jchapman @tlari @tsulaia

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