[ATR-23965] Auto configure input maker merging for empty steps (Vol.2)

follow up on @tamartin ’s branch !45624 (closed) Copying here the description, plus updates:

Aims to let the InputMaker in empty Steps auto-configure how it merges its inputs in empty steps.

In steps where there is such thing as a most-recent feature (i.e. empty steps which are appended on to pad the end of a leg), we should merge using the features to test equality.

In steps where there is no feature yet (i.e. empty steps which are prepended on to pad the start of a leg), we should merge using the only thing we can be 100% is accessible which is the "initiaRoI" link added by the HLT seeding algorithm.

Further improvements:

• To provide consistency and to save CPU, we now cache a hash of the ElementLink which we identify on each incoming Decision Object as the thing which we are using to match "identical" Decision Objects and hence perform the de-duplication from the filters.
• WARNING messages are raised if an individual IM uses both feature-matching AND roi-matching. It is tested happening in a combination of tau+jet+tau_probe chains without problems, but leave the WARNING to monitor other such cases
• Loosen the runtime validation checks. No longer require the "roi" link to be added in Empty IMs.
• One CI running caused ATLINFR-4329, debug messages are then removed.