Introduce tooling model for Android extensions plugin

Introduce Gradle models for plugins 'kotlin-android-extensions'.

libraries/tools/kotlin-gradle-plugin-model/src/main/java/org/jetbrains/kotlin/gradle/model/KotlinAndroidExtension.java

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.gradle.model;
+
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+/**
+ * Entry point for Kotlin Android Extensions models.
+ * Represents the description of Android only features. Provided by 'kotlin-android-extensions' plugin.
+ */
+public interface KotlinAndroidExtension {
+
+    /**
+     * Return a number representing the version of this API.
+     * Always increasing if changed.
+     *
+     * @return the version of this model.
+     */
+    long getModelVersion();
+
+    /**
+     * Returns the module (Gradle project) name.
+     *
+     * @return the module name.
+     */
+    @NotNull
+    String getName();
+
+    /**
+     * Indicate the use of experimental features.
+     *
+     * @return if experimental features are used.
+     */
+    boolean isExperimental();
+
+    /**
+     * Return the default cache implementation.
+     *
+     * @return the default cache implementation.
+     */
+    @Nullable
+    String getDefaultCacheImplementation();
+}