Adding metadata to information when calling chain.execute(string)

[mateusscheper]

The goal of this change is to allow the user to add extra information when appending the retrieved docs informations on chain.execute(string).
Example:

chain = ConversationalRetrievalChain.builder()               
                .metadata(List.of("file_name"))
                [...]
                .build();
String response = chain.execute("My question");

When building the prompt, it will include the information like:

information -> ...file_name: my_file.pdf
Content: my first retrieved content....

...file_name: my_file.pdf
Content: my second retrieved content...

Since it's my first contribution here, please let me know if something's wrong. Thanks!

[langchain4j]

@mateusscheper thanks a lot for your contribution! Please check my comments.

[langchain4j]

@mateusscheper looks great, thank you!