Add constructor with AuthenticationPolicy to ChatClient

[joseharriaga]

System.ClientModel's new AuthenticationPolicy makes it possible to use custom authentication mechanisms. For example, using Azure.Identity's DefaultAzureCredential, we can define a BearerTokenPolicy and use it to communicate with Azure OpenAI like this:

DefaultAzureCredential credential = new();
string scope = "https://cognitiveservices.azure.com/.default";
BearerTokenPolicy bearerTokenPolicy = new(credential, scope);

OpenAIClientOptions clientOptions = new()
{
    Endpoint = new Uri(Environment.GetEnvironmentVariable("AZURE_OPENAI_ENDPOINT"))
};

ChatClient client = new(
    model: "gpt-4.1-mini",
    authenticationPolicy: bearerTokenPolicy,
    options: clientOptions);

ChatCompletion completion = client.CompleteChat("Say 'this is a test.'");
Console.WriteLine($"[ASSISTANT]: {completion.Content[0].Text}");

[KrzysztofCwalina]

why are we doing this only in ChatCLient?

[joseharriaga]

why are we doing this only in ChatCLient?

@KrzysztofCwalina I wanted to start small. If this looks good, I will add it to everything else.