.Net Removing IPlan interface (#3265)

### Motivation and Context

IPlan interface has not definition and all Plan implementations actually
implement ISKFunction members.

Currently there is no need to have this as a `Flag interface`.

### Description

Removing `IPlan` interface and pointing all its dependencies to
`ISKFunction` directly.

### Contribution Checklist

- [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
- [ ] I didn't break anyone 😄

dotnet/src/SemanticKernel.Core/Planning/IPlan.cs

@@ -1,10 +1,13 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+using System;
+
 namespace Microsoft.SemanticKernel.Planning;
 
 /// <summary>
 /// Interface for standard Semantic Kernel callable plan.
 /// </summary>
+[Obsolete("This interface is obsoleted, use ISKFunction interface instead")]
 public interface IPlan : ISKFunction
 {
 }

dotnet/src/SemanticKernel.Core/Planning/InstrumentedPlan.cs

@@ -17,7 +17,7 @@ namespace Microsoft.SemanticKernel.Planning;
 /// <summary>
 /// Standard Semantic Kernel callable plan with instrumentation.
 /// </summary>
-internal sealed class InstrumentedPlan : IPlan
+internal sealed class InstrumentedPlan : ISKFunction
 {
     /// <inheritdoc/>
     public string Name => this._plan.Name;
@@ -31,10 +31,10 @@ internal sealed class InstrumentedPlan : IPlan
     /// <summary>
     /// Initialize a new instance of the <see cref="InstrumentedPlan"/> class.
     /// </summary>
-    /// <param name="plan">Instance of <see cref="IPlan"/> to decorate.</param>
+    /// <param name="plan">Instance of <see cref="Plan"/> to decorate.</param>
     /// <param name="loggerFactory">The <see cref="ILoggerFactory"/> to use for logging. If null, no logging will be performed.</param>
     public InstrumentedPlan(
-        IPlan plan,
+        ISKFunction plan,
         ILoggerFactory? loggerFactory = null)
     {
         this._plan = plan;
@@ -59,7 +59,7 @@ public async Task<FunctionResult> InvokeAsync(
 
     #region private ================================================================================
 
-    private readonly IPlan _plan;
+    private readonly ISKFunction _plan;
     private readonly ILogger _logger;
 
     /// <summary>

dotnet/src/SemanticKernel.Core/Planning/Plan.cs

@@ -22,7 +22,7 @@ namespace Microsoft.SemanticKernel.Planning;
 /// Plan is used to create trees of <see cref="ISKFunction"/>s.
 /// </summary>
 [DebuggerDisplay("{DebuggerDisplay,nq}")]
-public sealed class Plan : IPlan
+public sealed class Plan : ISKFunction
 {
     /// <summary>
     /// State of the plan

dotnet/src/SemanticKernel.Core/Planning/PlanExtensions.cs

@@ -1,5 +1,6 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+using System;
 using System.Linq;
 using Microsoft.Extensions.Logging;
 
@@ -73,7 +74,18 @@ public static string ToPlanString(this Plan plan, string indent = " ")
     /// </summary>
     /// <param name="plan">Instance of <see cref="IPlan"/> to decorate.</param>
     /// <param name="loggerFactory">The <see cref="ILoggerFactory"/> to use for logging. If null, no logging will be performed.</param>
+    [Obsolete("Use concrete class Plan WithInstrumentation instead")]
     public static IPlan WithInstrumentation(this IPlan plan, ILoggerFactory? loggerFactory = null)
+    {
+        throw new NotSupportedException("This method is obsolete, use concrete class Plan WithInstrumentation instead");
+    }
+
+    /// <summary>
+    /// Returns decorated instance of <see cref="ISKFunction"/> with plan enabled instrumentation.
+    /// </summary>
+    /// <param name="plan">Instance of <see cref="Plan"/> to decorate.</param>
+    /// <param name="loggerFactory">The <see cref="ILoggerFactory"/> to use for logging. If null, no logging will be performed.</param>
+    public static ISKFunction WithInstrumentation(this Plan plan, ILoggerFactory? loggerFactory = null)
     {
         return new InstrumentedPlan(plan, loggerFactory);
     }