setup TypeFactory for azure client plugin (Azure#45946)

* setup typefactory

* change the version to explicit instead of latest

* regen

* move the nullable option to a global place

Author: ArcturusZhang

eng/Packages.Data.props

@@ -242,7 +242,7 @@
   </ItemGroup>
 
   <ItemGroup Condition="'$(IsGeneratorLibrary)' == 'true'">
-    <PackageReference Update="Microsoft.Generator.CSharp.ClientModel" Version="1.0.0-alpha.20240904.4" />
+    <PackageReference Update="Microsoft.Generator.CSharp.ClientModel" Version="1.0.0-alpha.20240912.2" />
   </ItemGroup>
 
   <!--

eng/packages/http-client-csharp/generator/Azure.Generator/src/AzureClientPlugin.cs

@@ -3,6 +3,7 @@
 
 using Microsoft.Generator.CSharp;
 using Microsoft.Generator.CSharp.ClientModel;
+using System;
 using System.ComponentModel.Composition;
 
 namespace Azure.Generator;
@@ -14,12 +15,22 @@ namespace Azure.Generator;
 [ExportMetadata("PluginName", nameof(AzureClientPlugin))]
 public class AzureClientPlugin : ClientModelPlugin
 {
+    private static AzureClientPlugin? _instance;
+    internal static AzureClientPlugin Instance => _instance ?? throw new InvalidOperationException("AzureClientPlugin is not loaded.");
+
+    /// <inheritdoc/>
+    public override AzureTypeFactory TypeFactory { get; }
+
     /// <summary>
     /// The Azure client plugin to generate the Azure client SDK.
     /// </summary>
     /// <param name="context"></param>
     [ImportingConstructor]
-    public AzureClientPlugin(GeneratorContext context) : base(context) { }
+    public AzureClientPlugin(GeneratorContext context) : base(context)
+    {
+        TypeFactory = new AzureTypeFactory();
+        _instance = this;
+    }
 
     /// <summary>
     /// Customize the generation output for Azure client SDK.

eng/packages/http-client-csharp/generator/Azure.Generator/src/AzureTypeFactory.cs

@@ -0,0 +1,12 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+using Microsoft.Generator.CSharp.ClientModel;
+
+namespace Azure.Generator
+{
+    /// <inheritdoc/>
+    public class AzureTypeFactory : ScmTypeFactory
+    {
+    }
+}

eng/packages/http-client-csharp/generator/Directory.Build.props

@@ -1,6 +1,7 @@
 <Project ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <PropertyGroup>
     <IsClientLibrary>false</IsClientLibrary>
+		<Nullable>enable</Nullable>
   </PropertyGroup>
   <!--
     Add any shared properties you want for the projects under this package directory that need to be set before the auto imported Directory.Build.props