Make graphql-anywhere filter function generic

[minznerjosh]

A common part of many GraphQL/TypeScript setups is automatically generating TypeScript typings for any queries/mutations/fragments in the source code. For example, at Trialspark, we're using graphql-code-generator.

It would be great to more easily leverage the interfaces that tool is generating for our GraphQL fragments when using the filter() function from graphql-anywhere.

This PR makes filter() generic on two, optional type arguments. The first represents the filtered data that is returned; the second represents the larger superset of data that is passed. This way, we can pass the autogenerated typings for our fragments to the filter()s that use those fragments:

import { MyFragment, MyBigQuery } from './generated-types';

const FRAGMENT = gql`
  fragment MyFragment on Thing {
    id
    foo
  }
`
const lotsOfData = { /* pretend there's a lot here */};
const data = filter<MyFragment, MyBigQuery>(FRAGMENT, lotsOfData);

data.foo // fine
data.bar // not fine

[apollo-cla]

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[hwillson]

Great idea @minznerjosh - thanks!