Support for DynamoDB ReturnValues

I'm trying to update an item attribute in a DynamoDb table with AppSync. Once successful i want to use the previous value of that attribute as the key in my next call (I'm using pipeline resolvers).

To achieve this with DynamoDB you just set UPDATED_OLD as the return value.

I cannot find any documentation for specifying the return value of a DynamoDB resolver for AppSync. Ive tried the following.

```
{
"version" : "2018-05-29",
"operation" : "UpdateItem",
"key": {
"id": $util.dynamodb.toDynamoDBJson("foo")
},
"condition" : {
"expression" : "attribute_exists(id)"
},
"update" : {
"expression" : "SET bar = :bar",
"expressionValues" : {
":bar" : {"S" : "$bar"}
}
},
"returnValues": "UPDATED_OLD"
}
```
But its not valid syntax.

`"message": "Unsupported element '$[returnValues]'."
`

Start with the UpdateItem resolver example in the issue and compare AppSync's supported request mapping syntax with DynamoDB's ReturnValues options. Done means the supported way to request the previous updated attribute value is identified and documented, or the unsupported behavior is clearly confirmed.

Written by the indexing model from the issue text.