.Net: AgentKernelPluginFactory.CreateFromAgents - enable direct support for agents implicitly (#11443)

### Motivation and Context
1. Why
- it helps reduce cognitive overload and reduces the code to write, like
in python
   - solves #11439
2. Need to write too much code to set an agent as a plugin:
```
var agentPlugin = KernelPluginFactory.CreateFromFunctions("AgentPlugin",
            [
                AgentKernelFunctionFactory.CreateFromAgent(this.CreateSalesAssistant()),
                AgentKernelFunctionFactory.CreateFromAgent(this.CreateRefundAgent())
            ]);
```
while in Python:
`plugins=[billing_agent, refund_agent],`

3. Scenario
Making easier and simpler to write agentic code :)

4. Issue: #11439

### Description
Added the change as an extension method as KernelPluginFactory is in
SemanticKernel.Core without any reference to Agents.Core where the
method is. Unsure you would like to add a reference to it in the
SemanticKernel.Core, but it would be a lot more simpler to implement.

Now, the changes have been applied here:
https://github.com/microsoft/semantic-kernel/blob/0ff97c7fe73c5175df61ae627271341a32983c07/dotnet/samples/GettingStartedWithAgents/Step08_AgentAsKernelFunction.cs#L71

So it would look instead of the above code as:
```
        KernelPlugin agentPlugin = KernelPluginFactoryExtensions.CreateFromFunctions("AgentPlugin",
            [
                this.CreateSalesAssistant(),
                this.CreateRefundAgent()
            ]);
```
Which is a little less verbose, and more similar to the Python code.

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x ] The code builds clean without any errors or warnings
- [x ] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [ x] All unit tests pass, and I have added new tests where possible
- [x ] I didn't break anyone 😄

---------

Co-authored-by: Chris <[email protected]>
Co-authored-by: Mark Wallace <[email protected]>

Author: 3 people

dotnet/samples/GettingStartedWithAgents/Step08_AgentAsKernelFunction.cs

@@ -68,11 +68,12 @@ public async Task RefundAgent()
     public async Task MultipleAgents()
     {
         Kernel kernel = this.CreateKernelWithChatCompletion();
-        var agentPlugin = KernelPluginFactory.CreateFromFunctions("AgentPlugin",
+        KernelPlugin agentPlugin = AgentKernelPluginFactory.CreateFromAgents("AgentPlugin",
             [
-                AgentKernelFunctionFactory.CreateFromAgent(this.CreateSalesAssistant()),
-                AgentKernelFunctionFactory.CreateFromAgent(this.CreateRefundAgent())
+                this.CreateSalesAssistant(),
+                this.CreateRefundAgent()
             ]);
+
         kernel.Plugins.Add(agentPlugin);
         kernel.AutoFunctionInvocationFilters.Add(new AutoFunctionInvocationFilter(this.Output));
 

dotnet/src/Agents/Core/Functions/AgentKernelPluginFactory.cs

@@ -0,0 +1,47 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Linq;
+using Microsoft.SemanticKernel.Agents;
+
+namespace Microsoft.SemanticKernel;
+
+/// <summary>
+/// Extension methods for creating KernelPlugin instances from agents.
+/// </summary>
+[Experimental("SKEXP0110")]
+public static class AgentKernelPluginFactory
+{
+    /// <summary>
+    /// Creates a plugin from a collection of agents. Each agent is converted into a KernelFunction via AgentKernelFunctionFactory.
+    /// </summary>
+    /// <param name="pluginName">The name for the plugin.</param>
+    /// <param name="description">A description of the plugin.</param>
+    /// <param name="agents">A collection of agents to include in the plugin.</param>
+    /// <returns>A KernelPlugin with functions derived from the provided agents.</returns>
+    /// <exception cref="ArgumentNullException">Thrown when agents is null.</exception>
+    public static KernelPlugin CreateFromAgents(string pluginName, string? description, IEnumerable<Agent> agents)
+    {
+        if (agents == null)
+        {
+            throw new ArgumentNullException(nameof(agents));
+        }
+
+        KernelFunction[] functions = agents
+            .Select(agent => AgentKernelFunctionFactory.CreateFromAgent(agent))
+            .ToArray();
+
+        return KernelPluginFactory.CreateFromFunctions(pluginName, description, functions);
+    }
+
+    /// <summary>
+    /// Creates a plugin from an array of agents. Each agent is converted into a KernelFunction via AgentKernelFunctionFactory.
+    /// </summary>
+    /// <param name="pluginName">The name for the plugin.</param>
+    /// <param name="agents">The agents to include in the plugin.</param>
+    /// <returns>A KernelPlugin with functions derived from the provided agents.</returns>
+    public static KernelPlugin CreateFromAgents(string pluginName, params Agent[] agents) =>
+        CreateFromAgents(pluginName, description: null, agents);
+}