[ChatClient] Allow simple String prompts for convenient onboarding

[ThomasVitale]

Expected Behavior

The ChatClient provides a convenient and fluent API. For onboarding/getting-started/education scenarios, it would be nice to make it even more streamlined and allow simple prompts as String without the need to know about the concepts of message roles and user messages yet.

Something like this:

var response = chatClient.prompt("What did Gandalf say to the Balrog?")
                         .call()
                         .content();

That would be in line with the experience of using the Prompt class directly, since it provides a constructor that accepts a String:

var prompt = new Prompt("What did Gandalf say to the Balrog?");

Current Behavior

var response = chatClient.prompt()
                         .user("What did Gandalf say to the Balrog?")
                         .call()
                         .content();

[markpollack]

I like this.

[ThomasVitale]

Nice, thanks for the feedback! I have submitted a PR with the implementation for the suggested improvement> #1287