Types for input objects should not have a prototype

**Describe the bug**

The Javascript object that is returned for mutation inputs does not have a prototype but this is not reflected in the generated types.

See issue here: https://github.com/ardatan/graphql-tools/issues/1374

**To Reproduce**
Steps to reproduce the behavior:

2. My GraphQL operations:

```graphql
{
Mutation: {
someMutation(_, { input }) {
input.hasOwnProperty() // this will throw an error since input does not have a prototype
}
}
}
```

3. My `codegen.yml` config file:
```yml
overwrite: true
schema: '...'
documents: null
generates:
src/...
config:
noSchemaStitching: true
useIndexSignature: true
namingConvention:
enumValues: 'change-case#upperCase'
plugins:
- add:
content: '/* tslint:disable */'
- 'typescript'
- 'typescript-resolvers'
```

**Expected behavior**

Can we create a special type for object-inputs to null out all of their prototype fields?