Restore TVariables to useQuery return type

[keithlayne]

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

---

Problem

useQuery was clearly rewritten completely from 3.4 to 3.5, but its signature changed from

function useQuery<TData = any, TVariables = OperationVariables>(/*...*/): QueryResult<TData, TVariables>;

to

function useQuery<TData = any, TVariables = OperationVariables>(/*...*/): QueryResult<TData>;

This causes the TVariables type arg to be thrown away, and then defaulted to OperationVariables in QueryResult. This causes assignability issues in TS in some cases. The issue can be demonstrated with a trivial generic hook that wraps a query hook:

type DumbQuery = { foo: string };
type DumbVariables = { bar: number };
const DumbDocument = gql`
  query Dumb($bar: Int!) {
    dumb(bar: $bar) {
      foo
    }
  }
`;

function useDumbQuery(baseOptions: QueryHookOptions<DumbQuery, DumbVariables>) {
  const options = { ...baseOptions };
  return useQuery<DumbQuery, DumbVariables>(DumbDocument, options);
}

// Trivially wrapping a query hook
const useDumbWrappedQuery = <Q, V>(
  hook: (options: QueryHookOptions<Q, V>) => QueryResult<Q, V>,
  options: QueryHookOptions<Q, V> = {},
) => hook(options);

const Test = () => {
  useDumbWrappedQuery(useDumbQuery);
  /*
  Argument of type '(baseOptions: QueryHookOptions<DumbQuery, DumbVariables>) => QueryResult<DumbQuery, OperationVariables>' is not assignable to parameter of type '(options: QueryHookOptions<DumbQuery, DumbVariables>) => QueryResult<DumbQuery, DumbVariables>'.
  Call signature return types 'QueryResult<DumbQuery, OperationVariables>' and 'QueryResult<DumbQuery, DumbVariables>' are incompatible.
    The types of 'variables' are incompatible between these types.
      Type 'OperationVariables | undefined' is not assignable to type 'DumbVariables | undefined'.
        Type 'OperationVariables' is not assignable to type 'DumbVariables'.
  */

  return null;
};

Notes

• This problem doesn't appear when all of the properties of TVariables are optional.
• The "testing" box isn't checked because I don't know if you have any test infrastructure for types, and this is a type-only problem. I didn't look too hard, so if I missed it, let me know.
• npm run build and npm run test worked fine with this change.

@brainkim 🚎 👀

[apollo-cla]

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[brainkim]

Keith, have I ever told you you are my fourth favorite cat among cats that I know? Thank you!