ATR-19379 Consistent use of DecisionID typedef. Consistent use of ElementLinkVector "seed" in R3 trigger steering.

This MR covers two topics. Topic 1

• Following on from !20600 (merged) and !20959 (merged), we are now more sensitive to type differences. In this case, that DecisionIDs are uint32_t but some places of the code handled them as int. Now, DecisionID is typedef'd in exactly one location and aliased throughout.
• Given how common the vector<DecisionID> = decisions is for the Run3 trigger, this is added as an explicit AUX variable to the newly added V2 aux container (cc @bernius) rather than being a near universal decoration.

Topic 2

• In performing the above I noted that some tidy up was needed in TrigCompositeUtils. cc @tbold, @fpastore, @sgeorge. We should be consistent about if seed is a single ElementLink or is a collection of element links in a ElementLinkVector. As it seems that the latter is required, I have changed the various methods which interact with the seed to handle many seeds. This might be the cause of some strange crashes I'm seeing so triggering a round of CI on this one.

(Touching TUT for unit tests)