Add production dependency on @types/node-fetch. (apollographql/apol…

…lo-server#3546)

* Add production dependency on `@types/node-fetch`.

As the `HeadersInit` is exported and used in the exported [`GatewayConfig`]
type via [`RemoteGatewayConfig`] - which is emitted to `@apollo/gateway`'s
`dist/index.d.ts` declaration, the `node-fetch` types need to be available
by those who install the package.

[`GatewayConfig`]: https://github.com/apollographql/apollo-server/blob/73cdf0d533908894282debdb6dc5444186af27a6/packages/apollo-gateway/src/index.ts#L74-L77
[`RemoteGatewayConfig`]: https://github.com/apollographql/apollo-server/blob/73cdf0d533908894282debdb6dc5444186af27a6/packages/apollo-gateway/src/index.ts#L62

Fixes: apollographql/apollo-server#3471

* Add CHANGELOG.md for apollographql/apollo-server#3546.

Apollo-Orig-Commit-AS: apollographql/apollo-server@c63786b

gateway-js/package.json

@@ -20,6 +20,7 @@
   },
   "dependencies": {
     "@apollo/federation": "file:../apollo-federation",
+    "@types/node-fetch": "2.5.4",
     "apollo-engine-reporting-protobuf": "file:../apollo-engine-reporting-protobuf",
     "apollo-env": "^0.6.0",
     "apollo-graphql": "^0.3.4",