fix: handle union type response

[chrstnmcf]

The graphql-composer was not able to handle union type return types so when a query/mutation had a union type response (e.g. UpdateAuthorAddressResponse) while collecting the queries the graphql composer tried to access for example the field UpdateAuthorAddressResponse.id instead of Author.id which does not exist.

updateAuthorAddress(authorId: $authorId, address: $address) {
  __typename
  ...on Author {
    id name { firstName lastName }
  }
  ... on NotFoundError {
    message
  }
}

So I added a check if the return type is a union type and if this is the case it passes the type name of the fragment to collectNestedQueries so that when collecting the nested queries for fragments, it knows the actual field type name.

Also in order to correctly build the queries again in the end a new fragment property was added to the query node selection so that when building the query for a union type response it wraps the selection with the fragment (e.g. ... on Author { ... }.

I added some test cases for this and this solution worked perfectly fine for our use cases so I hope I haven't missed anything.

[mcollina]

lgtm