[FEATURE] Return retrieved Contents/TextSegments when using AI Service with RAG

[KaisNeffati]

Context

#660

Change

If you wish to access the sources (retrieved Contents used to augment the message),
you can easily do so now by wrapping the return type in the Result class:

interface Assistant {

    Result<String> chat(String userMessage);
}

Result<String> result = assistant.chat("How to do Easy RAG with LangChain4j?");

String answer = result.content();
List<Content> sources = result.sources();

Checklist

Before submitting this PR, please check the following points:

• I have added unit and integration tests for my change
• All unit and integration tests in the module I have added/changed are green
• All unit and integration tests in the core and main modules are green
• I have added/updated the documentation
• I have added an example in the examples repo (only for "big" features)
• I have added my new module in the BOM (only when a new module is added)

Checklist for adding new embedding store integration

• I have added a {NameOfIntegration}EmbeddingStoreIT that extends from either EmbeddingStoreIT or EmbeddingStoreWithFilteringIT

[langchain4j]

@KaisNeffati thank you a lot for your contribution!

[langchain4j]

Hi @KaisNeffati, do you mind if I will do and push some changes?

[KaisNeffati]

Feel free to do it @langchain4j

[langchain4j]

@KaisNeffati could you please add me as collaborator to your repo? I can't push to this branch/PR :(

[KaisNeffati]

Done

[langchain4j]

@KaisNeffati thank you! I have pushed my changes, could you please review? I hope now it is a bit more generic (Result instead of WithSources). I have also changed UserMessage to ChatMessage right away in all places to avoid doing breaking changes one more time in the near future.

[KaisNeffati]

Just a minor comment below !

And Yeah, Result is a very wise choice. It confirms what I had in mind initially when I thought about 'ResponsePrompt' or 'ResponseMessage'. I'm glad to hear that you've found the right name 👏

Atherwise , all is good for me

[langchain4j]

@KaisNeffati thanks! Then I am merging it.