Stub generation infrastructure (except for metadata generation) for KAPT+K2

This commit includes the basic Java stubs generation infrastructure and
the corresponding tests. The main entry point is called
Kapt4StubGenerator. Calls to it from production code will be added in a
separate commit.

 #KT-51982

plugins/kapt3/kapt3-compiler/testData/converter/interfaceImplementation.fir.txt

@@ -0,0 +1,30 @@
+@kotlin.Metadata()
+public abstract interface Named {
+
+    @org.jetbrains.annotations.Nullable()
+    public abstract java.lang.String getName();
+}
+
+////////////////////
+
+
+@kotlin.Metadata()
+public final class Product2 implements Named {
+    @org.jetbrains.annotations.Nullable()
+    private java.lang.String name = null;
+
+    @org.jetbrains.annotations.Nullable()
+    @java.lang.Override()
+    public java.lang.String getName() {
+        return null;
+    }
+
+    public void setName(@org.jetbrains.annotations.Nullable()
+    java.lang.String p0) {
+    }
+
+    public Product2(@org.jetbrains.annotations.NotNull()
+    java.lang.String otherName) {
+        super();
+    }
+}