Add option_type field within OptionValue type

[waiting-for-dev]

This allows to retrieve option value values along their option type
name, description, etc. E.g.:

query getProduct {
  productBySlug(slug: "solidus-t-shirt") {
    name
    description
    variants {
      nodes {
        id
        sku
        position
        prices {
          nodes {
            amount
            currency {
              htmlEntity
            }
          }
        }
        optionValues {
          nodes {
            id
            name
            presentation
            optionType {
              name
              position
              presentation
            }
          }
        }
      }
    }
  }
}

Fixes #171

[kennyadsl]

@waiting-for-dev do you get why codeclimate is complaining?

[waiting-for-dev]

@waiting-for-dev do you get why codeclimate is complaining?

As part of the code to be pushed is auto-generated, it helped decrease the ratio of tested/non-tested code on my push. I added a test to be compliant. I don't think they're super-useful, but it's consistent with the approach taken in the extension, so it's fine by now.