Add Advisor to Automatically Apply Structured Output Formatting

[damienmiheev]

Currently, to apply a StructuredOutputConverter format to a prompt, I need to manually include a {format} placeholder inside the template and pass it as a variable:

StructuredOutputConverter outputConverter = ...

String userInputTemplate = """
    ... user text input ...
    {format}
    """;

Prompt prompt = new Prompt(
    PromptTemplate.builder()
        .template(userInputTemplate)
        .variables(Map.of("format", outputConverter.getFormat()))
        .build()
        .createMessage()
);

This puts the responsibility on the developer to modify the prompt template just to enable structured output. It would be more ergonomic if the formatting could be applied automatically, e.g., via an Advisor or another abstraction that augments the prompt consistently.

Ideally, the Advisor mechanism could be generalized to support vendor-specific structured response formats (e.g., OpenAI JSON Schema / response format specs) without requiring template placeholders.