Added HttpClient Injection

[tdechanterac]

I've added some constructor to be able to inject the HttpClient into the OpenAi class.
This allow to customize the HttpClient further such as adding a retry policy, or adding delegating handler to the HttpClient.
I also updated the readme file to show how we can use it in a simple case.

[darkwatchuk]

Hi,

Think you've missed passing the httpClient on the ChatEndpoint...

internal ChatEndpoint(HttpClient httpClient, OpenAIAPI api) : base(api) { }

should be this?

internal ChatEndpoint(HttpClient httpClient, OpenAIAPI api) : base(httpClient,api) { }