Update example code blocks

Author: seesharprun

learn-pr/azure/build-dotnet-app-azure-cosmos-db-nosql/includes/4-exercise-create-resources.md

@@ -207,6 +207,7 @@ Your app now creates a database and container. The methods you used to create th
 1. Review the *Program.cs* code file to make sure that your code matches this sample:
 
     ```csharp
+    using Azure.Identity;
     using Microsoft.Azure.Cosmos;
     using Microsoft.Azure.Cosmos.Fluent;
     using Microsoft.Azure.Cosmos.Linq;

learn-pr/azure/build-dotnet-app-azure-cosmos-db-nosql/includes/5-exercise-create-items.md

@@ -249,6 +249,7 @@ Your app now creates multiple items and is designed to be resilient enough to be
 1. Review the *Program.cs* code file to make sure that your code matches this sample.
 
     ```csharp
+    using Azure.Identity;
     using Microsoft.Azure.Cosmos;
     using Microsoft.Azure.Cosmos.Fluent;
     using Microsoft.Azure.Cosmos.Linq;

learn-pr/azure/build-dotnet-app-azure-cosmos-db-nosql/includes/6-exercise-read-query.md

@@ -176,6 +176,7 @@ Your app now reads and queries items from the container. Here, you run the appli
 1. Review the *Program.cs* code file to make sure that your code matches this sample.
 
     ```csharp
+    using Azure.Identity;
     using Microsoft.Azure.Cosmos;
     using Microsoft.Azure.Cosmos.Fluent;
     using Microsoft.Azure.Cosmos.Linq;

learn-pr/azure/build-dotnet-app-azure-cosmos-db-nosql/includes/7-exercise-linq.md

@@ -117,6 +117,7 @@ Your app now uses LINQ to build a cross-partition query that your developer team
 1. Review the *Program.cs* code file to make sure that your code matches this sample.
 
     ```csharp
+    using Azure.Identity;
     using Microsoft.Azure.Cosmos;
     using Microsoft.Azure.Cosmos.Fluent;
     using Microsoft.Azure.Cosmos.Linq;