Make checksum optional (#912)

When using serialization frameworks like pydantic, I can not attach a consumer record because checksum can be None.  Since it is marked as deprecated, it seems reasonable to mark it as optional here.

Author: zschumacher

aiokafka/structs.py

@@ -81,7 +81,7 @@ class ConsumerRecord(Generic[KT, VT]):
     value: Optional[VT]
     "The value"
 
-    checksum: int
+    checksum: Optional[int]
     "Deprecated"
 
     serialized_key_size: int