[Streaming] Streaming Python API (#6755)

streaming/python/message.py

@@ -0,0 +1,17 @@
+class Record:
+    """Data record in data stream"""
+
+    def __init__(self, value):
+        self.value = value
+        self.stream = None
+
+    def __repr__(self):
+        return "Record(%s)".format(self.value)
+
+
+class KeyRecord(Record):
+    """Data record in a keyed data stream"""
+
+    def __init__(self, key, value):
+        super().__init__(value)
+        self.key = key