Allow GlobalChi2Fitter to create track summaries

As we move towards fitters (possibly) returning tracks with fitters, we will need some infrastructure for track fitters to return summaries. This simple commit adds a flag to the Trk::GlobalChi2Fitter that makes it return empty track summaries in addition to the tracks themselves.

This merge request builds on !36404 (merged). Now that that merge request is merged, this change should have no impact on the output quality, while still laying the groundwork for infrastructure for creating track summaries in fitters.