Types for AppSync lambda resolver don't match

Hi, I'm using AppSync + GraphQL + Lambda Resolvers through Amplify (v8.3.0). I'm using TypeScript to write my lambda resolver, but the type of `event` seems slightly different from the definition for `AppSyncResolverEvent` in [`@types/aws-lambda`](https://github.com/DefinitelyTyped/DefinitelyTyped/blob/ee1993e96c9e4a5c335361b1a2c7cb706bc9d4ef/types/aws-lambda/trigger/appsync-resolver.d.ts#L47).

The actual event object looks like:
```
{
"typeName": "Mutation",
"fieldName": "createInitialMedia",
"arguments": {
"projectId": "07a46207-19fb-439c-91a8-490e09dc86f7"
},
"identity": {
...
},
"source": null,
"request": {
...
},
"prev": {
"result": {}
}
}
```

Is `AppSyncResolverEvent` the wrong type to be using here? Or is this event object format specific to Amplify?

`schema.graphl` for reference:

```
type Mutation {
createInitialMedia(projectId: String!): Media
@function(name: "mediaApi-${env}")
}
```

Compare the Amplify AppSync Lambda event shown in the issue with AppSyncResolverEvent in @types/aws-lambda/trigger/appsync-resolver.d.ts at line 47, using schema.graphl as context. Determine whether the declared type or the reported event shape is incorrect, and document or correct the matching type definition.

Written by the indexing model from the issue text.