Simplify the implementation of ChatClient-based interfaces

[jxblum]

Provide sensible defaults for overloaded methods in PromptUserSpec, PromptSystemSpec, AdvisorSpec, CallResponseSpec, ChatClientRequestSpec, and Builder interfaces.

[markpollack]

I'd like to keep the interfaces without any defaults as in WebClient and other spring projects. This PR also introduces a conflict in that the single implementation of the existing spec classes have different behavior it seems at first glance. I took a look at PromptUserSpec for example.

@Override
public PromptUserSpec text(String text) {
	Assert.hasText(text, "text cannot be null or empty");
	this.text = text;
	return this;
}

vs.

default PromptUserSpec text(String text) {
	Charset defaultCharset = Charset.defaultCharset();
	return text(new ByteArrayResource(text.getBytes(defaultCharset)), defaultCharset);
}

What is wrong with using the string as supplied i the current impl?

In anycase, these impls are used in one place, so if there are changes to make in the impl, i'd do it there and not default method interfaces.