fix(codegen-schema): map String list to String list in input type

[craicoverflow]

Fixes #2203

Types of changes

What types of changes does your code introduce to Graphback?

• Bugfix (non-breaking change which fixes an issue)

Checklist

• I have read the CONTRIBUTING document.
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Verification

Given model:

"""@model"""
type ModelWithArrayScalar {
  id: ID!
  roles: [String!]!
}

This will create the input type:

input MutateModelWithArrayScalar {
  id: ID!
  roles: [String]
}

Further comments