Implement unions in GQL

[magento-engcom-team]

Implement unions in graphql

As a developer I want to be able to implement unions in the Magento graphql schema language following the specification https://spec.graphql.org/June2018/#sec-Unions

Acceptance criteria:
allow a syntax like the following:

union CompanyStructureEntity 
@doc(description: "some kind of union") 
@typeResolver(class: "Magento\\Module\\Resolver")  =
    CompanyTeam | Customer
{code}



Allow and resolve a query with proper types representing each type in union like the following:
{code:java}
{
  company {
    structure {
      items {
        id
        parent_id
        entity {
          __typename
          ... on Customer {
            firstname
            lastname
          }
          ... on CompanyTeam {
            name
          }
        }
      }
    }
  }
}

[magento-engcom-team]

The issue was exported from the internal JIRA. The link to the original JIRA issue: https://jira.corp.magento.com/browse/MC-36456

[m2-assistant]

Hi @magento-engcom-team. Thank you for your report.
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