Allow to configure non-default JDK using toolchain via extension.

Add method into "kotlin" extension to set toolchain for all tasks.
This will also set toolchain for java compile tasks. Also user may
set toolchain via "java" extension and it will be applied to Kotlin
tasks as well.

^KT-43092 In Progress

libraries/tools/kotlin-gradle-plugin-api/src/main/kotlin/org/jetbrains/kotlin/gradle/tasks/UsesKotlinJavaToolchain.kt

@@ -11,6 +11,7 @@ import org.gradle.api.tasks.Input
 import org.gradle.api.tasks.Internal
 import org.gradle.api.tasks.Nested
 import org.gradle.jvm.toolchain.JavaLauncher
+import org.gradle.util.GradleVersion
 import java.io.File
 
 interface UsesKotlinJavaToolchain {
@@ -66,4 +67,8 @@ interface KotlinJavaToolchain {
             javaLauncher: Provider<JavaLauncher>
         )
     }
+
+    companion object {
+        val TOOLCHAIN_SUPPORTED_VERSION: GradleVersion = GradleVersion.version("6.7")
+    }
 }