Orleans 7 Stateless Worker Grains Not Available in IManagementGrain.GetDetailedGrainStatistics

After upgrading to Orleans 7, Stateless Worker Grains are no longer available in `IManagementGrain.GetDetailedGrainStatistics` call (was available in Orleans 3.x). Could not find any indication of whether this is a desired behaviour, so it seems like an regression.

After taking a brief look under the hood, it seems like a filtering took place in `Orleans.Runtime.Catalog.GetDetailedGrainStatistics`, where Stateless Worker Grains in `ActivationDirectory activations` has their `GrainInstance` set to null, thus being filtered out from the results of the parent call.

Start with IManagementGrain.GetDetailedGrainStatistics and Orleans.Runtime.Catalog.GetDetailedGrainStatistics, then trace how ActivationDirectory activations are filtered when GrainInstance is null. Compare the Orleans 7 behavior with Orleans 3.x and determine whether Stateless Worker Grains should appear in the results; done means the expected behavior is confirmed and covered by regression evidence.

Written by the indexing model from the issue text.