Migration Report/Summary

**Is your feature request related to a problem? Please describe.**
The logging we currently have is extensive and detailed. This is great for understanding exactly what happened but is too complicated for most of our user base.

**Describe the solution you'd like**
It would be nice if the engine returned a summary of what happened.

- How many rows were read/written per entity?
- How many failures? What were they?
- How many warnings? What were they?
- How long did it take per entity?

This should be a typed object that the various interfaces can surface accordingly (e.g. the XrmToolBox visually displays).