Default plan resolver should use `.get(...)` if possible
<!-- Before opening an issue: if you use `npm` try running `yarn` instead; if
      you run `yarn`, please try running `npx yarn-deduplicate && yarn`. -->

### Summary

When I try to wrap a step result in `object` I get an error:

```
Error: The step returned by 'createAsset.asset' is not compatible with the GraphQL object type 'Asset': Expected a PgSelectSingleStep, PgInsertSingleStep, PgUpdateSingleStep or PgDeleteSingleStep, however we received 'Access{2}➊<35.asset>[42]'.
```

### Steps to reproduce

A code example:

```ts
makeExtendSchemaPlugin((build) => {
   const { assets } = build.input.pgRegistry.pgResources;
   if (!assets) throw new Error("...");

   return {
     typeDefs: gql`
       type CreateAssetPayload {
         asset: Asset!
       }

       CreateAssetInput {
        ...
       }
  
       extend type Mutation {
         createAsset(input: CreateAssetInput!): CreateAssetPayload
       }
     `,
     plans: {
       Mutation: {
         createAsset() {
           const $id = withPgClient(...);

           return object({ asset: assets.get({ id: $id }) });
         }
       }
     }
   }
});
```

There is no problem when it returns `assets.get({ id: $id })`

The issues will be fixed if you add the following plan:

```ts
CreateAssetPayload: { 
    asset($payload) { 
        return $payload.get(‘asset’);
    } 
}
```

### Expected results

You don't have to add the `CreateAssetPayload` plan resolver

### Additional context

My PostGraphile version is `5.0.0-beta.35`
