Add the ability to configure a custom output parser

[anunnakian]

Issue

#968

Change

Add the ability to configure a custom output parser, here is an example :

First of all, I define City class and my custom output parser by implementing OutputParser interface

@Data
@AllArgsConstructor
static class City {
    private String name;
}

static class CityOutputParser implements OutputParser<City> {

    @Override
    public City parse(String string) {
        return new City(string);
    }

    @Override
    public String formatInstructions() {
        return "A name of a city";
    }
}

Then, I use my custom output parser by adding an annotation on my method called @Output like this :

interface AiService {

        @Output(CityOutputParser.class)
        @UserMessage("What is the capital of Germany?")
        City chat();
}

Here is a test :

@Test
void test() {

    // given
    AiService aiService = AiServices.builder(AiService.class)
            .chatLanguageModel(chatLanguageModel)
            .build();

    // when-then
    assertThat(aiService.chat()).isEqualTo(new City("Berlin"));
    verify(chatLanguageModel).generate(singletonList(userMessage("What is the capital of Germany?A name of a city")));
}

General checklist

• There are no breaking changes
• I have added unit and integration tests for my change
• I have manually run all the unit and integration tests in the module I have added/changed, and they are all green
• I have manually run all the unit and integration tests in the core and main modules, and they are all green

• I have added/updated the documentation
• I have added an example in the examples repo (only for "big" features)

Checklist for adding new model integration

• I have added my new module in the BOM

Checklist for adding new embedding store integration

• I have added a {NameOfIntegration}EmbeddingStoreIT that extends from either EmbeddingStoreIT or EmbeddingStoreWithFilteringIT
• I have added my new module in the BOM

Checklist for changing existing embedding store integration

• I have manually verified that the {NameOfIntegration}EmbeddingStore works correctly with the data persisted using the latest released version of LangChain4j

[langchain4j]

Related: #1176