Support specifying data source groups when using the Engine

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.