.Net: AzureOpenAI with DefaultAzureCredential sample

[markwallace-microsoft]

Motivation and Context

Sample showing how to use Azure Open AI Chat Completion with Azure Default Credential.

Contribution Checklist

• [ ] The code builds clean without any errors or warnings
• [ ] The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• [ ] All unit tests pass, and I have added new tests where possible
• [ ] I didn't break anyone :smile: