Azure OpenAI: non-Azure OpenAI API key should be a strongly-typed credential object

[trrwilson]

PR #35045 added the capability to use Azure.AI.OpenAI's OpenAIClient to connect to the non-Azure OpenAI endpoint using an API key from https://platform.openai.com.

This is currently represented as a plain string, but needs to be updated to a strongly-typed credential object that derives from TokenCredential and mirrors the functionality of AzureKeyCredential.

Additionally, when adding this new type:

• Appropriate client builder overloads (AddOpenAIClient()) should be added
• The use of client builder should be documented, including a link to the overall dependency injection documentation: https://learn.microsoft.com/en-us/dotnet/azure/sdk/dependency-injection

[annelo-msft]

@trrwilson, can this be closed now that we have the new ClientModel APIs for this?

[jmrtnz94]

Dropping this incase anyone else is trying to figure out how to register this with the ClientModel APIs

builder.Services.AddAzureClients(azBuilder =>
{
    azBuilder.AddClient<OpenAIClient, OpenAIClientOptions>(options =>
        new OpenAIClient(builder.Configuration.GetValue<string>("MY_OPEN_AI_KEY"), options));
});