Support specifying data source groups when using the Engine

[mslukebo]

Is your feature request an entirely new concept?
Yes,

Is your feature request related to an existing component?
It is related to feature request #213

Is your feature request related to a problem? Please describe.
With the proposed changes in #213, engine users have no way to specify which groups returned by an IDataSourceGrouper get used during processing.

Describe the solution you'd like
A new interface IDataSourceGroupResolver is created for picking out the IDataSourceGroups to use for processing. It would look like

public interface IDataSourceGroupResolver
{
        IReadOnlyCollection<IDataSourceGroup> Resolve(Guid processingSourceGuid, IReadOnlyCollection<IDataSourceGroup> allValidGroups);
}

Then, EngineCreateInfo would have a new instance method WithResolver(IDataSourceGroupResolver resolver) that sets the IDataSourceGroupResolver to use when the engine decides which groups to actually use for processing.