[jhw] DynamoDb Local support

[jweissBHG]

It's possible to run DynamoDB locally (see https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/DynamoDBLocal.html). Just needed to be able to pass in an override to the default Endpoint calculated for the AwsClient.

I was looking into supporting DAX with this change, but although the documentation for DAX makes it seem like "it's the same API as Dynamo, it's a write-through cache", DAX actually uses a binary protocol (CBOR) over TCP, and the C# implementation is not openly available as far as I can tell. The GO one is, and it's pretty clear this would not be a small undertaking, so I don't plan on doing it anymore. https://github.com/aws/aws-dax-go