Refactor ReceiveMessageHandler

Introduce PlainTextWriter to improve indentation handling.

src/main/java/org/asamk/signal/PlainTextWriter.java

@@ -0,0 +1,23 @@
+package org.asamk.signal;
+
+import java.io.IOException;
+
+public interface PlainTextWriter {
+
+    void println(String format, Object... args) throws IOException;
+
+    PlainTextWriter indentedWriter();
+
+    default void println() throws IOException {
+        println("");
+    }
+
+    default void indent(final WriterConsumer subWriter) throws IOException {
+        subWriter.consume(indentedWriter());
+    }
+
+    interface WriterConsumer {
+
+        void consume(PlainTextWriter writer) throws IOException;
+    }
+}