Improve Erlang 19's Dialyzer results for erlcloud_ddb2

Make it possible for Dialyzer (Erlang 19) to conclude generic {error, term()}
errors can be returned on misc. erlcloud_ddb2 calls.
If we don't explicitly declare that #ddb2_error{} fields might be undefined,
Dialyzer will mark the DDBError binding initialization on
erlcloud_ddb_impl:request_and_retry/4 (line 198 as of 988528d) as invalid
due to all the missing fields. This, in its turn, invalidates analysis of
the remaining pipeline of result processing, which in its turn blinds Dialyzer
to the fact that errors other than "{'socket_error',_}" and "{'http_error',_,_,_}}"
can and will be returned (e.g. conditional check failures.)

include/erlcloud_ddb2.hrl

@@ -1,14 +1,14 @@
 -record(ddb2_error,
-        {attempt :: pos_integer(),
-         error_type :: ddb | http | httpc,
-         should_retry :: boolean(),
-         reason :: term(),
-         request_headers :: [{string(), string()}],
-         request_body :: jsx:json_text(),
-         response_status :: pos_integer(),
-         response_status_line :: string(),
-         response_headers :: [{string(), string()}],
-         response_body :: binary()
+        {attempt :: undefined | pos_integer(),
+         error_type :: undefined | ddb | http | httpc,
+         should_retry :: undefined | boolean(),
+         reason :: undefined | term(),
+         request_headers :: undefined | [{string(), string()}],
+         request_body :: undefined | jsx:json_text(),
+         response_status :: undefined | pos_integer(),
+         response_status_line :: undefined | string(),
+         response_headers :: undefined | [{string(), string()}],
+         response_body :: undefined | binary()
         }).
 
 -type date_time() :: number().