AWSLambdaEvents.AppSync.Event incorrect

### Expected behavior
I am using swift as a appsync lambda resolver with the following GraphQL schema.

```GraphQL
input TestInput {
ids: [String]
}

testMutation(input: TestInput!): String
```
According to the [GraphQL Documentation](https://graphql.org/learn/schema/) arrays are allowed. So a correct JSON Decode should happen.

### Actual behavior
By setting the `message` to type of `AWSLambdaEvents.AppSync.Event` leads to a `Unexpected AppSync argument. Expected a String or a Dictionary.` error.

### Steps to reproduce

```Json
{
"field": "testMutation",
"arguments": {
"input": {
"ids": [
"id1",
"id2"
]
}
}
}
```
This is a the json payload I get from appsync.

### Swift & OS version (output of `swift --version && uname -a`)
Apple Swift version 5.4 (swiftlang-1205.0.26.9 clang-1205.0.19.55)
Target: x86_64-apple-darwin20.4.0
Darwin xx-MacBook-Pro.local 20.4.0 Darwin Kernel Version 20.4.0: Thu Apr 22 21:46:47 PDT 2021; root:xnu-7195.101.2~1/RELEASE_X86_64 x86_64

Start by locating AWSLambdaEvents.AppSync.Event and its JSON decoding path, then reproduce the issue with the payload shown in the report. Trace why nested arrays such as arguments.input.ids are rejected; done means the payload decodes successfully without the Unexpected AppSync argument error, with coverage for this case if the project has relevant decoder tests.

Written by the indexing model from the issue text.