K2 kapt: add kapt.use.k2 Gradle property

... and `-Kapt-use-k2` CLI flag to enable K2 kapt.

 #KT-61114 Fixed

plugins/kapt3/kapt3-base/src/org/jetbrains/kotlin/kapt3/base/KaptOptions.kt

@@ -129,7 +129,8 @@ enum class KaptFlag(val description: String, val defaultValue: Boolean = false)
     INCREMENTAL_APT("Incremental annotation processing (apt mode)"),
     STRIP_METADATA("Strip @Metadata annotations from stubs"),
     KEEP_KDOC_COMMENTS_IN_STUBS("Keep KDoc comments in stubs", defaultValue = true),
-    USE_JVM_IR("Use JVM IR backend", defaultValue = true)
+    USE_JVM_IR("Use JVM IR backend", defaultValue = true),
+    USE_K2("Use K2"),
     ;
 }