Add getContent() method to ChatResponse

[habuma]

Simple convenience method on ChatRespoonse to return the content of the first generation.

Although ChatResponse has many useful bits of information carried in it, the most common thing needed by all applications is the content of the first generation. E.g.

String content = chatResponse.getResult().getOuptut().getContent();

Because it's the most commonly needed information from a ChatResponse, it would be helpful if there were a convenience method that returns the content directly instead of having to navigate through a few other properties. That's what this PR provides. The above code can then be replaced with:

String content = chatResponse.getContent();

[markpollack]

the branch modelcaller has a new fluent api that covers this use case and more.