chore: Fix TS errors related to FieldSets

[trevor-scheer]

We have some overlapping types like FederationType and FederationTypeMetadata
where properties like keys and requires should always be a FieldSet.

This commit simplifies the code a bit and adds assertions + casts where
necessary to ensure these types match as we intend them to.

How these types have been out of sync for so long is surprising. I've
noticed them in the past but it's never been a blocking issue until
I was working on some code with @o0Ignition0o and we ran into these
TS errors.

In any case I consider this to be a low-lift improvement worth landing,
even though I suppose there are some types we could combine to
simplify this a bit further.

The long-existing errors I'm referring to:

federation-js/src/composition/DirectiveMetadata.ts:187:13 - error TS2322: Type 'FederationField' is not assignable to type 'FederationFieldMetadata'.

187             federation: fieldFederationMetadata,
                ~~~~~~~~~~

federation-js/src/composition/compose.ts:547:13 - error TS2322: Type 'FederationField' is not assignable to type 'FederationFieldMetadata'.
  Types of property 'requires' are incompatible.
    Type 'readonly SelectionNode[] | undefined' is not assignable to type 'FieldSet | undefined'.
      Type 'readonly SelectionNode[]' is not assignable to type 'readonly (FieldNode | InlineFragmentNode)[]'.
        Type 'SelectionNode' is not assignable to type 'FieldNode | InlineFragmentNode'.
          Type 'FragmentSpreadNode' is not assignable to type 'FieldNode | InlineFragmentNode'.
            Type 'FragmentSpreadNode' is not assignable to type 'FieldNode'.
              Types of property 'kind' are incompatible.
                Type '"FragmentSpread"' is not assignable to type '"Field"'.

547             federation: fieldFederationMetadata,
                ~~~~~~~~~~

federation-js/src/composition/compose.ts:573:11 - error TS2322: Type 'FederationField' is not assignable to type 'FederationFieldMetadata'.

573           federation: fieldFederationMetadata,
              ~~~~~~~~~~

federation-js/src/composition/compose.ts:595:13 - error TS2322: Type 'FederationField' is not assignable to type 'FederationFieldMetadata'.

595             federation: fieldFederationMetadata,

[martijnwalraven]

Yep, this definitely seems like a sensible fix.

I think the reason these differences haven't been a problem before is that the only time these packages are compiled together is during testing, because ts-jest doesn't support per-project TypeScript configuration. In other cases, these types exist in isolated projects and shouldn't affect each other.

That's why compilation with tsc (as we do in npm run compile or before tests) works fine, but running tests spits out these errors. (And it only does that the first time you run tests it seems, because ts-jest caches the results).

Somewhat related, parseFieldSet has always been a bit of a hack, because we're forced to parse a document and extract the selections. Recent versions of graphql-js expose Parser, so we should be able to start parsing with the right parsing rule directly (as is done for existing utility methods like parseValue). Not sure when we're ready to drop support for older versions of graphql-js though.

[trevor-scheer]

@martijnwalraven good tip about the Parser class! We already require ^15.5.3 so I went ahead and made that change.

[glasser]

@martijnwalraven does that mean that after this change we may be able to simplify/improve something about the Jest/TS setup in this repo?