Env variable for default key buffer size (#4001)

* - introduce environment variable for DEFAULT_KEY_BUFFER_SIZE

* - prefix env variable with MOTO_S3 to avoid env variable conflicts

* - black formatting

* - fix formatting

moto/settings.py

@@ -21,3 +21,7 @@ def get_sf_execution_history_type():
         returned. Default value is SUCCESS, currently supports (SUCCESS || FAILURE)
     """
     return os.environ.get("SF_EXECUTION_HISTORY_TYPE", "SUCCESS")
+
+
+def get_s3_default_key_buffer_size():
+    return int(os.environ.get("MOTO_S3_DEFAULT_KEY_BUFFER_SIZE", 16 * 1024 * 1024))