Validate [5.3.2] Field Selection Merging

[Leonardo Losoviz]

As required by the GraphQL spec: https://spec.graphql.org/draft/#sec-Field-Selection-Merging

It is difficult to implement, as it must happen in MirrorQueryDataStructureFormatter (as decided on runtime, not based on static analysis), and we don't keep a reference to the Field that set the value in the response, as to compare later on that any overriding field (eg: because it has the same alias) is different (here):

$resolvedObjectRet[$leafField->getOutputKey()] = $resolvedObject[$leafField];

Started working on branch Field-Selection-Merging-Validation, but left it unfinished.

[Leonardo Losoviz]

Actually it can be implemented, as the list of all the fields to resolved for each object as passed all together as $fields in method addObjectData, so there's no need to keep track of the fields after printing their values, and the validation can be performed in advance.