Feature Request: Option to return a type for single entity mutations as opposed to just the entity itself

Hi Hasura team,

At the moment the single entity mutations do not return an object with a typename:
```graphql
mutation($object: tests_insert_input!) {
insert_tests_one(object: $object){
id
}
}
```

Feature request: ability to return an object with a typename of e.g., `__insert_tests_one_response` (as opposed to something that does not describe the type of mutation like `tests_mutation_response`)

```graphql
mutation($object: tests_insert_input!) {
insert_tests_one(object: $object){
__typename
node {
id
}
}
}
```

Use case: client cache invalidation based on types :)

I know that just returning the type itself is useful, so maybe this could be an option?

Thanks for considering!

Lydia