Support customising response templates for integration with DynamoDB

[balan-jayavictor]

Without this, the response from DynamoDB for GetItem calls would look like this:
{ "Key1": "[{"S":"Value1"}]", "Key2": "[{"S":"Value2-1"},{"S":"Value2-2"},{"S":"Value2-3"},{"S":"Value2-4"},{"N": 25},{"S":"Value2-6"}]" }