.NET: [BREAKING] Workflows API Review Naming Changes (Part 1?) (#4090)

* refactor: Normalize Run/RunStreaming with AIAgent

* refactor: Clarify Session vs. Run -level concepts

* Rename RunId to SessionId to better match Run/Session terminology in AIAgent
* [BREAKING]: Will break existing checkpointed sessions in CosmosDb due to field rename

* refactor: Rename and simplify interface around getting typed data out of ExternalRequest/Response

* Also adds hints around using value types in PortableValue

* refactor: Rename AddFanInEdge to AddFanInBarrierEdge

This will prevent a breaking change later when we introduce a programmable FanIn edge, analogous to the FanOut edge's EdgeSelector.

The goal, in the long run is to support a number of different FanIn scenarios, with naive FanIn (no barrier) by default, similar to FanOut.

* refactor: AsAgent(this Workflow, ...) => AsAIAgent(...)

* misc - part1: SwitchBuilder internal

---------

Co-authored-by: Dmytro Struk <13853051+dmytrostruk@users.noreply.github.com>

dotnet/samples/GettingStarted/Workflows/SharedStates/Program.cs

@@ -27,7 +27,7 @@ public static class Program
         // Build the workflow by connecting executors sequentially
         var workflow = new WorkflowBuilder(fileRead)
             .AddFanOutEdge(fileRead, [wordCount, paragraphCount])
-            .AddFanInEdge([wordCount, paragraphCount], aggregate)
+            .AddFanInBarrierEdge([wordCount, paragraphCount], aggregate)
             .WithOutputFrom(aggregate)
             .Build();