fix: implement InMemoryLogStorage.append

This method was newly added to LogStorage to support appends of
unserialized application data.

The implementation of the test class ListLogStorage seemed like a
reasonable solution for zeebe-process-test. It simply writes the
contents of the writer into a buffer and appends it as usual.

See:
https://github.com/camunda/zeebe/blob/4dd1025f3f3ba86b5aec10d380b9db8550ed0967/logstreams/src/test/java/io/camunda/zeebe/logstreams/util/ListLogStorage.java#L77-L79

engine/src/main/java/io/camunda/zeebe/process/test/engine/InMemoryLogStorage.java

@@ -9,6 +9,7 @@
 
 import io.camunda.zeebe.logstreams.storage.LogStorage;
 import io.camunda.zeebe.logstreams.storage.LogStorageReader;
+import io.camunda.zeebe.util.buffer.BufferWriter;
 import java.nio.ByteBuffer;
 import java.util.ArrayList;
 import java.util.HashSet;
@@ -33,6 +34,17 @@ public LogStorageReader newReader() {
     return new ListLogStorageReader();
   }
 
+  @Override
+  public void append(
+      final long lowestPosition,
+      final long highestPosition,
+      final BufferWriter bufferWriter,
+      final AppendListener listener) {
+    final var buffer = ByteBuffer.allocate(bufferWriter.getLength());
+    bufferWriter.write(new UnsafeBuffer(buffer), 0);
+    append(lowestPosition, highestPosition, buffer, listener);
+  }
+
   @Override
   public void append(
       final long lowestPosition,