[GraphQl] Support for unions and interfaces for search functionality

I am trying to create a general search functionality in my app that can return multiple types of objects. For each type of object I want to retrieve different fields.

In the graphQl specification this is supported using interfaces and unions. See this example below:
```GraphQl
type Query {
# Search across all content
search(query: String!): [SearchResult]
}

union SearchResult = Conference | Festival | Concert | Venue
```

```GraphQl
query {
search(query: "Madison") {
... on Venue {
id
name
address
}

... on Festival {
id
name
performers
}

... on Concert {
id
name
performingBand
}

... on Conference {
speakers
workshops
}
}
}
```
See [this guide](https://docs.aws.amazon.com/appsync/latest/devguide/interfaces-and-unions.html) for more details.

I tried making all my entities extend a 'base' entity and creating the search query on that entity, but that did not work.

Thanks in advance!