.Net: New Feature: Expose UserAgent header customization

[gaeaehrlich]

The Azure SDK enables to append a string to the UserAgent header, it is being done by the OpenAIClientOptions.Diagnostics.ApplicationId. We need from SK to expose the same capability, perhaps by adding a new property to the ChatCompletionAgent Execution Settings.

In our implementation we don't have access to the user provided HTTP client, but even if we find a way to get the HTTP client it won't help, because SK and azure SDK add user agent header on the request and it will override the client default header.