mCBM2021 digi MQ chain, with ts source + unpacker + event builder + event sink

Also contains related changes.

Following commits will be dropped from the MR before removing the Draft tag:

• p.-a.loizeau/cbmroot@b1cf2044 will be handled once !622 (closed) is merged
• p.-a.loizeau/cbmroot@70fb4f11 will need its own MR
• p.-a.loizeau/cbmroot@cfbbce9a will probably also be better handled in an independent MR
• p.-a.loizeau/cbmroot@c58d8ef4 may be partially handled in an independent MR (the changes in the sim/transport/steer folder)

Once these are out the branch will go through an interactive rebase to compress the commits to "working units"