Consider variable default values with @skip/@include/@defer

[BoD]

Related to #4784

In the response adapters, we generate lines like this:

if (not(variable("skipIf")).evaluate(customScalarAdapters.adapterContext.variables(), __typename, customScalarAdapters.adapterContext, null)) {
  // Read fields for the fragment
}

but customScalarAdapters.adapterContext.variables() was empty, since it only considered variables that are not optional, or optional and present.

with this approach we include the optionals that are absent and have a default value (booleans only)

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[BoD]

Just pushed:

• use variable adapters instead of introducing a new variables() method (and use a flag that piggybacks on CustomScalarAdapters.AdapterContext)
• revert what was done in Fix normalizing data when using @include or @skip with default values #4700 for the case of the normalizer - the new approach works for it too

[martinbonnin]

👏

[BoD]

⚠️ Just a reminder that tweaks will be needed to avoid breaking changes when backporting to 3.x