Support Kotlin/Multiplatform Gradle plugin natively

[Tapchicoma]

It would be nice if Spring-boot Gradle plugin could support natively Kotlin/Multiplatform ("org.jetbrains.kotlin.multiplatform") Gradle plugin:

• Kotlin 2.1.20 is prohibiting Gradle java, java-library and application plugins in combination with Kotlin multiplatform plugin if a project is using Gradle 8.7+. With previous Gradle versions such combination produces a warning. (issue)
• For application plugin replacement new experimental API similar to the Gradle one was added (issue)
• Kotlin multiplatform plugin for a several releases already by default always applies java-base Gradle plugin
  Spring boot plugin currently expects that Gradle java plugin is present in the project to do the main configuration

The possible ways to solve it - either to react on java-base plugin or create a special plugin action for "org.jetbrains.kotlin.multiplatform" plugin.

[wilkinsona]

either to react on java-base plugin

As I understand it, the base plugin only sets up conventions, it doesn't actually create anything itself. Spring Boot's plugin relies on several things that are created by the java plugin such as the main source set and various configurations so I don't think reacting to the java-base plugin would meet our needs.

create a special plugin action for "org.jetbrains.kotlin.multiplatform" plugin

I think this is what we'll have to do. There may be some common logic that we can extract from our existing JavaPluginAction but I think the entry point will have to be reacting to org.jetbrains.kotlin.multiplatform.