Round 2 of cleanup for agent runtime and orchestrations (#164)

- Moved InProcessRuntime type into abstractions package and deleted InProcess package.
- Moved several members of IAgentRuntime to be extension methods instead, e.g. multiple GetActorAsync overloads.
- Added synchronous RegisterMessageHandler overloads and used them to avoid unnecessary async usage at call sites.
- Removed unnecessary surface area from InProcessRuntime, e.g. StopAsync, RunUntilIdleAsync, etc.
- Fixed spin loop in InProcessRuntime that would consume an entire core for the duration of the orchestration's operation.
- Removed a bunch of allocation from InProcessRuntime.
- Made a runtime optional for orchestrations, defaulting to using a temporary InProcessRuntime if none is provided.
- Removed custom delegate types from orchestrations.
- Consolidated namespaces.
- Used records to simplify message classes.
- Tweaked naming on AgentActor to make purpose of protected methods more clear.
- Removed invocation in AgentActor.InvokeAsync of empty update / isFinal parameter.
- Changed OrchestrationHandoffs to avoid needing to pass in agents duplicatively.
- Made various extension methods, such as those on OrchestrationHandoffsExtensions, into instance methods.

dotnet/samples/GettingStarted/Orchestration/ConcurrentOrchestration_With_StructuredOutput.cs

@@ -2,10 +2,7 @@
 
 using System.Text.Json;
 using Microsoft.Agents.Orchestration;
-using Microsoft.Agents.Orchestration.Concurrent;
-using Microsoft.Agents.Orchestration.Transforms;
 using Microsoft.Extensions.AI.Agents;
-using Microsoft.Extensions.AI.Agents.Runtime.InProcess;
 using Microsoft.Shared.Samples;
 
 namespace Orchestration;
@@ -43,20 +40,14 @@ public class ConcurrentOrchestration_With_StructuredOutput(ITestOutputHelper out
                 ResultTransform = outputTransform.TransformAsync,
             };
 
-        // Start the runtime
-        await using InProcessRuntime runtime = new();
-        await runtime.StartAsync();
-
         // Run the orchestration
         const string resourceId = "Hamlet_full_play_summary.txt";
         string input = Resources.Read(resourceId);
         Console.WriteLine($"\n# INPUT: @{resourceId}\n");
-        OrchestrationResult<Analysis> result = await orchestration.InvokeAsync(input, runtime);
+        OrchestrationResult<Analysis> result = await orchestration.InvokeAsync(input);
 
-        Analysis output = await result.GetValueAsync(TimeSpan.FromSeconds(ResultTimeoutInSeconds * 2));
+        Analysis output = await result;
         Console.WriteLine($"\n# RESULT:\n{JsonSerializer.Serialize(output, s_options)}");
-
-        await runtime.RunUntilIdleAsync();
     }
 
 #pragma warning disable CA1812 // Avoid uninstantiated internal classes