Prevent multiple SG lookups from same tracking tool

SiCombinatorialTrackFinder_xk retrieves pixel and SCT clusters from StoreGate many times in a single execution of the FastTrackFinder.

Instead, I have added a cache for SG pointers to the existing cache object SiCombinatorialTrackFinderData_xk.

Since this object has the scope of a single call to the algorithm execute() method, the cache should be valid: https://acode-browser.usatlas.bnl.gov/lxr/source/athena/Trigger/TrigAlgorithms/TrigFastTrackFinder/src/TrigFastTrackFinder.cxx#0535

Overall effect should just be a small performance improvement.