Appsync: Support in-line access of properties on the result of a service call

[sam-goodwin]

Support the following access pattern:

return table.get(..).property

Currently, this mistakenly renders a Velocity Template where the RequestMappingTemplate ends with:

$util.toJson($request).property

It should actually translate to an expression the ResponseMappingTemplate:

#set($context.stash.return__val = $context.result.property)

[sam-goodwin]

@thantos I think #280 solved this one? Can we close?

[thantos]

Yep!