Allow orchestrations to send external events to other orchestrations

[cgillum]

Today one can use a DurableOrchestrationClient to raise events to a running orchestration instance. However, it would be nice if orchestrations could raise events to other orchestrations as well. Starting with DurableTask.Core v2.1.0, this will be possible, and should be exposed to Durable Functions.

[cfe84]

Arguably you can just use an activity to do so ; but that would be awesome yes.