Expose additional metadata that's currently internal/private

[stephentoub]

Confirm this is not a feature request for the underlying OpenAI API.

• This is not a feature request for the underlying OpenAI API

Confirm this is not a feature request for Azure OpenAI.

• This is not a feature request for Azure OpenAI

Describe the feature or improvement you're requesting

• OpenAIClient should expose Url Endpoint { get; }
• ChatClient should expose string Model { get; }
• Clients like ChatClient should expose a way to navigate back to the owning OpenAIClient so that if you have access to the ChatClient, you can access its Endpoint, or alternatively the Endpoint should also be exposed on ChatClient.

Additional context

No response