ApolloServer + geteway disable initial request, or handle auth error

it appears that ApolloServer + ApolloGeteway will make an initial request to each endpoint.

in a scenario where we need to get authorization token from a user request, this initial call will fail.

in this case, the first call fails, and ApolloServer no longer sends any requests to that url.

would it be possible to either avoid the first request, or send it somewhere else (when auth token is not available) ? alternatively maybe we can handle the error so that future request still go out

```
const gateway = new ApolloGateway({
serviceList: [
{ name: 'svc1', url: svc1url }, {...}
],
buildService({ name, url }) {
return new AuthenticatedDataSource({ url });
},
});

const server = new ApolloServer({
gateway,
subscriptions: false,
context: (ctx:ExpressContext) => {
return {auth: ctx.req.header('authorization')}
}
});
```

No file or test is named in the issue. Start with the ApolloGateway serviceList/buildService path and the AuthenticatedDataSource shown in the example; reproduce an initial request without an authorization header and determine the expected behavior for subsequent requests. Done means the chosen behavior is specified and covered by a regression test.

Written by the indexing model from the issue text.