Feature request: Support for the Long datatype (64 bit integer)

DynamoDB's "Number" type supports large numbers that can be represented with Long.

From the docs https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/HowItWorks.NamingRulesDataTypes.html:

```
Number
Numbers can be positive, negative, or zero. Numbers can have up to 38 digits precision. Exceeding this results in an exception.

Positive range: 1E-130 to 9.9999999999999999999999999999999999999E+125

Negative range: -9.9999999999999999999999999999999999999E+125 to -1E-130
```

So if we are getting data from a DynamoDB table via AppSync, if there's a field that is of a "Long" type, AppSync should support a corresponding data type that can parse and serialize the data properly, instead of throwing an error.
```
Can't serialize value (/queryItems/LongProperty) : Expected type 'Int' but was 'Long’.
```

Start with the AppSync GraphQL path that maps DynamoDB Number values, and review the linked DynamoDB data-type documentation and the discussion on this issue. Confirm how Long values are currently parsed and serialized, then verify that a Long-valued field no longer produces the reported Expected type 'Int' error.

Written by the indexing model from the issue text.