backfill @required on ChangeMessageVisibilityBatchRequestEntry.VisibilityTimeout

[lucix-aws]

relates aws/aws-sdk#527
closes #2250

This is a stopgap to unblock customers affected by this issue. An upstream modeling fix still needs to be pursed with the SQS team.

Fix issue where SDK could not send a VisibilityTimeout of 0 in a ChangeMessageVisibilityBatchRequestEntry.

        &sqs.ChangeMessageVisibilityBatchInput{
            QueueUrl: aws.String("foo"),
            Entries: []types.ChangeMessageVisibilityBatchRequestEntry{
                {   
                    Id:            aws.String("bar"),
                    ReceiptHandle: aws.String("baz"),
                },  
            },  
        },

before

{"Entries":[{"Id":"bar","ReceiptHandle":"baz"}],"QueueUrl":"foo"}

after

{"Entries":[{"Id":"bar","ReceiptHandle":"baz","VisibilityTimeout":0}],"QueueUrl":"foo"}

[Madrigal]

LGTM, should we add a unit test to verify when can we actually get rid of this if/when a fix is added upstream?