.NET: Add AgentWorkflowBuilder group chat (#861)

* Add AgentWorkflowBuilder group chat

And fix a variety of issues along the way:
- Use DateTime{Offset}.UtcNow rather than Now
- AIAgentHostExecutor shouldn't be publishing empty messages
- Sequential workflows should be flowing all history and not just the output from the previous agent as the input into the next agent
- Renamed some of the new agent workflow methods... still not super happy with the shape, though
- Simplified handoffs builder, e.g. using a hashset with a custom comparer instead of a dictionary
- Improved multi-service use by trying to change assistant->user role for messages created by other agents
- Changed MessageMerger to rely on M.E.AI's coalescing more and to avoid empty contents / text
- Ensured that messages from ChatClientAgent include MessageId and CreatedAt timestamps
- Avoided including instructions for agents in a handoff workflow that don't have any handoffs
- Removed the unnecessary end function in handoffs
- Improved naming of executors to include agent name for debuggability
- Use "N" formatting with Guid.ToString everywhere, to avoid the unnecessary extra dash character which is also not valid in various places (like function tool names)
- Replace `params T[]` with `params IEnumerable<T>` to make public APIs more flexible in what they consume

* Address feedback

- Fix unintentional provider change in sample

dotnet/samples/GettingStarted/AgentOpenTelemetry/Program.cs

@@ -123,7 +123,7 @@ appLogger.LogInformation("Agent created successfully with ID: {AgentId}", agent.
 
 // Create a parent span for the entire agent session
 using var sessionActivity = activitySource.StartActivity("Agent Session");
-var sessionId = Guid.NewGuid().ToString();
+var sessionId = Guid.NewGuid().ToString("N");
 sessionActivity?
     .SetTag("agent.name", "OpenTelemetryDemoAgent")
     .SetTag("session.id", sessionId)