feat: add Producer superclass above VideoProducer

This pvaes the way for parts of the pipeline that do not produce videos,
such as JSON, images, clips etc

TODO: rename module video_producers → producers

pipeline/video_producers.py

@@ -7,10 +7,15 @@ import tempfile
 
 class VideoProducer(ABC):
     """Video producer interface."""
+class Producer(ABC):
+    """Generic producer interface."""
     def __init__(self, features):
-        pass
+        """All producers should take a list of features as input"""
     def produce(self):
-        pass
+        """All Producers should produce something!"""
+
+class VideoProducer(Producer):
+    """Video producer interface."""
 
 
 class FfmpegVideoProducer(VideoProducer):