Migrate GlobalChi2Fitter to use new conditions architecture (ATLASRECTS-5441)

The GlobalChi2Fitter tool was previously interacting directly with the old-style magnetic field service. For reasons of thread safety, this is no longer the recommended way of retrieving information about the magnetic field. Instead, we should access this information through magnetic field cache objects from the new conditions architecture. This commit replaces the old-style magnetic field code with new-style code by adding a magnetic field cache data member to the fitter's internal cache class, adding logic to initialize this magnetic field cache on an event-by-event basis, as well as by modifying the logic that previously accessed the service directly.

Closes ATLASRECTS-5441.