Allow the repack expanding limit to be configurable

As seen in https://gitlab.cern.ch/cta/operations/-/issues/1100, there is an hardcoded constraint that limits the number of ongoing repack expansions to 2 (link: https://gitlab.cern.ch/cta/CTA/-/blob/d97fed4b/scheduler/Scheduler.cpp#L425).

As discussed in the dev meeting of 30/06/2023, we want to make this value configurable on the tape server configuration file.

The default value should remain 2.