Add a flag to disable the fieldsCanMerge validation on disjoint types

[martinbonnin]

Given this schema:

  type Query {
    foo: PropertyValue
  }
  union PropertyValue = PropertyValueBoolean | PropertyValueInt

  type PropertyValueBoolean {
    name: String!
    value: Boolean
  }
  type PropertyValueInt {
    name: String!
    value: Int
  }

and this query:

  query GetFoo {
    foo {
      ...PropertyValue
    }
  }
  fragment PropertyValue on PropertyValue {
    __typename
    ... on PropertyValueBoolean {
        value
    }
    ... on PropertyValueInt {
        value
    }
  }

Validation fails with:

`value` cannot be merged with `value`: they have different shapes. 

This is expected from reading the spec although it's not clear what the benefit of this validation is on completely disjoint types and it makes the generated models more akward to use.

Other users have bumped into the same issue in other projects like here

We could have a Gradle flag that specifically disables this validation:

apollo {
  fieldOnDisjointTypesMustMerge = false
}

More context:

• Spec pull request: RFC: Return type overlap validation graphql/graphql-spec#162
• Slack discussion: https://kotlinlang.slack.com/archives/C01A6KM1SBZ/p1659536084099869

[pt2121]

Can I give this a try?

[martinbonnin]

Very sorry for the delay @pt2121 ! Thanks for looking into this. I'm reviewing the PR now, I'll follow up there.

[martinbonnin]

This is now available in 3.6.0