Spring-ai Simplify implementation of Usage in OpenAI support

Now there is an implementation of Usage named OpenAiUsage

public class OpenAiUsage implements Usage {

    public static OpenAiUsage from(OpenAiApi.Usage usage) {
        return new OpenAiUsage(usage);
    }

    private final OpenAiApi.Usage usage;

This should not contain the Azure SDK class as a private field member. DefaultUsage should be used instead.

See #814 for some original context on this issue.