.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/Workflows/SharedStates/Program.cs

@@ -62,7 +62,7 @@ internal sealed class FileReadExecutor() : ReflectingExecutor<FileReadExecutor>(
         // Read file content from embedded resource
         string fileContent = Resources.Read(message);
         // Store file content in a shared state for access by other executors
-        string fileID = Guid.NewGuid().ToString();
+        string fileID = Guid.NewGuid().ToString("N");
         await context.QueueStateUpdateAsync(fileID, fileContent, scopeName: FileContentStateConstants.FileContentStateScope);
 
         return fileID;