.Net: Move OpenApi extensions to the SemanticKernel namespace. (#8906)

### Motivation, Context, and Description
The OpenAPI extensions for Kernel and RestApiOperationResponse classes
are are moved to the Microsoft.SemanticKernel namespace to follow the
same approach taken by other Import* extensions like
ImportPluginFromType, ImportPluginFromObject etc.

Closes: #6882

Author: SergeyMenshykh

dotnet/samples/Concepts/Planners/HandlebarsPlanning.cs

@@ -3,7 +3,6 @@
 using Microsoft.SemanticKernel;
 using Microsoft.SemanticKernel.Connectors.OpenAI;
 using Microsoft.SemanticKernel.Planning.Handlebars;
-using Microsoft.SemanticKernel.Plugins.OpenApi;
 using Plugins.DictionaryPlugin;
 using Resources;
 using xRetry;

dotnet/samples/Concepts/RAG/WithPlugins.cs

@@ -6,7 +6,6 @@
 using Microsoft.SemanticKernel.Connectors.Chroma;
 using Microsoft.SemanticKernel.Connectors.OpenAI;
 using Microsoft.SemanticKernel.Memory;
-using Microsoft.SemanticKernel.Plugins.OpenApi;
 using Resources;
 
 namespace RAG;

dotnet/src/Functions/Functions.OpenApi/Extensions/OpenApiKernelExtensions.cs

@@ -9,8 +9,9 @@
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Abstractions;
 using Microsoft.SemanticKernel.Http;
+using Microsoft.SemanticKernel.Plugins.OpenApi;
 
-namespace Microsoft.SemanticKernel.Plugins.OpenApi;
+namespace Microsoft.SemanticKernel;
 
 /// <summary>
 /// Provides extension methods for importing plugins exposed as OpenAPI specifications.

dotnet/src/Functions/Functions.OpenApi/Extensions/RestApiOperationResponseExtensions.cs

@@ -4,7 +4,7 @@
 using System.Text.Json;
 using Json.Schema;
 
-namespace Microsoft.SemanticKernel.Plugins.OpenApi;
+namespace Microsoft.SemanticKernel;
 
 /// <summary>
 /// Class for extensions methods for the <see cref="RestApiOperationResponse"/> class.

dotnet/src/Functions/Functions.UnitTests/OpenApi/RestApiOperationResponseTests.cs

@@ -1,7 +1,6 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using Microsoft.SemanticKernel;
-using Microsoft.SemanticKernel.Plugins.OpenApi;
 using SemanticKernel.Functions.UnitTests.OpenApi.TestResponses;
 using Xunit;