.NET: Consider supporting aggregation across the FanInBarrier edge

It seems there is occasional confusion about the "fan-in barrier" edge, wherein users expect it to aggregate the incoming messages. This is likely further confused by the use of FanIn when defining ChatProtocol-based workflows, which tend to pass around and handle `List`, leading to issues like #3724.

We should consider adding explicit support for aggregation across the edge type, though we need to be careful about how to handle cases where upstream outputs may have multiple, mutually incompatible types.

The work here may result in the decision not to make changes.