Support keyless authentication with Azure OpenAI

Best practice guidance is to use keyless authentication with Azure resources, and we'll soon publish this guidance in our AI docs as well. We should update the demo to use a recommended pattern for this. For example, here's the default approach, using the Azure.Identity library, that is included in the upcoming doc:

```csharp
Kernel kernel = Kernel
.CreateBuilder()
.AddAzureOpenAITextGeneration(
"your-model",
"your-endpoint",
new DefaultAzureCredential())
.Build();
```

This does increase some complexity in configuring the application, which for other samples we include in the infra code that's published with AZD. Perhaps there's a good practice for this with Aspire already, I just haven't dived in there yet - Filing the issue for now so I don't forget.