Introduce Kotlin Serialization auto-configuration

[dmitrysulman]

This PR adds auto-configuration support for Kotlin Serialization in Spring Boot, addressing spring-projects/spring-boot#44241

Summary of Changes

1. New Module: Introduced a new module, spring-boot-kotlin-serialization, which provides KotlinSerializationAutoConfiguration. This auto-configuration supports a set of new properties under the spring.kotlin-serialization prefix for customizing the Json instance from kotlinx.serialization.
2. New Starter: Added a corresponding starter module spring-boot-starter-kotlin-serialization.
3. HTTP Message Converters Integration:

• Introduced KotlinSerializationHttpMessageConvertersConfiguration in spring-boot-http-converter.
• Added support for a new kotlin-serialization option to the spring.http.converters.preferred-json-mapper property to expose KotlinSerializationJsonHttpMessageConverter as a bean.
• Updated the HttpMessageConverters configuration to add KotlinSerializationJsonHttpMessageConverter immediately before MappingJackson2HttpMessageConverter. This restores the behavior prior to the Spring Framework change when spring.http.converters.preferred-json-mapper=kotlin-serialization is explicitly set.

4. Documentation: Updated relevant JSON feature documentation to include the new kotlin-serialization option and configuration support.
5. Test Coverage: All newly introduced logic is fully covered by tests.

There was a suggestion in the original issue to use kotlinx as the option name. However, kotlinx is a general namespace used across many Kotlin libraries (like kotlinx.coroutines, kotlinx-datetime, kotlinx-io), not just for serialization. To avoid confusion and be more specific, I chose to use kotlin-serialization both in the code (KotlinSerialization* classes) and as the value for the spring.http.converters.preferred-json-mapper property. This keeps the naming clear and consistent.

Let me know if anything needs to be changed or improved, happy to adjust things if needed.

Fixes #44241

[dmitrysulman]

This is a workaround to prevent the CheckAutoConfigurationClasses task from failing. The issue is that the Kotlin Gradle Plugin adds a kotlin SourceDirectorySet to the main SourceSet even when there are no Kotlin source files. As a result, the CheckAutoConfigurationClasses task throws an IOException when it tries to walk through the /build/classes/kotlin/main directory, which doesn't exist.

[dmitrysulman]

This is probably not the ideal solution, it would be better to address the issue in the CheckAutoConfigurationClasses task itself by filtering out the Kotlin directory from the scan.

[dmitrysulman]

@bclozel just wanted to check if this PR reflects what was proposed in #44241? I’d be glad to adjust it if something needs to be changed.

[bclozel]

Thanks for your contribution @dmitrysulman , this is now merged in the main branch and will be released with our next milestone.