Add additional integration tests and add OpenAIAssistant integration tests project. (#79)

* Add additional integration tests and add OpenAIAssistant integration tests project.

* Address PR comments.

* Address PR comments.

dotnet/src/Shared/IntegrationTests/OpenAIConfiguration.cs

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+// Copyright (c) Microsoft. All rights reserved.
+
+namespace Shared.IntegrationTests;
+
+#pragma warning disable CS8618 // Non-nullable field must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring as nullable.
+#pragma warning disable CA1812 // Internal class that is apparently never instantiated.
+
+internal sealed class OpenAIConfiguration
+{
+    public string? ServiceId { get; set; }
+
+    public string ChatModelId { get; set; }
+
+    public string ApiKey { get; set; }
+}

dotnet/src/Shared/IntegrationTests/README.md

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+# Integration Tests
+
+Common Integration test files.
+
+To use this in your project, add the following to your `.csproj` file:
+
+```xml
+<PropertyGroup>
+  <InjectSharedIntegrationTestCode>true</InjectSharedIntegrationTestCode>
+</PropertyGroup>
+```