DynamoDbEnhancedClient: Support for scan rate limiting

From https://github.com/aws/aws-sdk-java-v2/issues/35:

> With the current DynamoDBMapper implementation, when performing scans, there's no easy way to make the scan operation respect the DynamoDB table's read throughputs.
Solutions combining https://aws.amazon.com/blogs/developer/rate-limited-scans-in-amazon-dynamodb/ and https://aws.amazon.com/blogs/developer/understanding-auto-paginated-scan-with-dynamodbmapper/ have to be used and are not that straightforward.

> Please provide a scan operation that is able to respect a table's read throughput (whether by explicitly specifying a percentage of throughput to use or an absolute value of read units to use).