This PR makes the Kotlin DSL generate accessors for elements of properties of extensions that are containers.

For example this makes the DSL for JVM test suites more discoverable and terse.

Before:

testing {
    suites {
        named<JvmTestSuite>("test") {
            useJUnit()
        }
    }
}

After:

testing {
    suites {
        test {
            useJUnit()
        }
    }
}

• Fixes Properties of extensions that are containers should have Kotlin DSL accessors for existing elements #9264

To support use cases that are valuable for this work such as JVM test suites and Android build types, the polymorphic containers now honor the declared public types of their elements in their schema. They do so in a way that the schema is stable whichever is the state of their elements, realised or not: the base element type of container elements is preferred over their concrete type.

To support this a new @PublicType annotation is introduced to statically declare preferred public types of DSL objects. This will replace the old HasPublicType interface that requires realised instances causing changing collection schemas when elements get realised.

This PR doesn't change the accessors exposed for tasks that expose more concrete types than other polymorphic containers, it also doesn't change how its schema is unstable after realising its elements. This should be fixed eventually but is out of scope of this PR.

• See Handle public types correctly and uniformly in polymorphic containers #23740