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_Intro.cs

@@ -1,9 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using Microsoft.Agents.Orchestration;
-using Microsoft.Agents.Orchestration.Concurrent;
 using Microsoft.Extensions.AI.Agents;
-using Microsoft.Extensions.AI.Agents.Runtime.InProcess;
 
 namespace Orchestration;
 
@@ -42,20 +40,14 @@ public class ConcurrentOrchestration_Intro(ITestOutputHelper output) : Orchestra
                 StreamingResponseCallback = streamedResponse ? monitor.StreamingResultCallback : null,
             };
 
-        // Start the runtime
-        await using InProcessRuntime runtime = new();
-        await runtime.StartAsync();
-
         // Run the orchestration
         string input = "What is temperature?";
         Console.WriteLine($"\n# INPUT: {input}\n");
-        OrchestrationResult<string[]> result = await orchestration.InvokeAsync(input, runtime);
+        OrchestrationResult<string[]> result = await orchestration.InvokeAsync(input);
 
-        string[] output = await result.GetValueAsync(TimeSpan.FromSeconds(ResultTimeoutInSeconds));
+        string[] output = await result;
         Console.WriteLine($"\n# RESULT:\n{string.Join("\n\n", output.Select(text => $"{text}"))}");
 
-        await runtime.RunUntilIdleAsync();
-
         this.DisplayHistory(monitor.History);
     }
 }