Rename IChatClient's methods and result types

[SteveSandersonMS]

Outcome from a team discussion

Before:

public interface IChatClient
{
    Task<ChatCompletion> CompleteAsync(...);
    IAsyncEnumerable<StreamingChatCompletionUpdate> CompleteStreamingAsync(...);
}

After:

public interface IChatClient
{
    Task<ChatResponse> GetResponseAsync(...);
    IAsyncEnumerable<ChatResponseUpdate> GetStreamingResponseAsync(...);
}

Of course, this will also affect all the many overloads of CompleteAsync and CompleteAsync<T>.

[stephentoub]

I like the outcome we reached. I think that's much cleaner and not as tied to underlying mechanics.