Integrating SOFA with Apollo Federation

I have a microservice API that is using SOFA and seems to be serving up data via REST just fine:

```
const { ApolloServer } = require('apollo-server-express');
const { buildFederatedSchema } = require('@apollo/federation');
const express = require('express');
const { useSofa } = require('sofa-api');

const { resolvers } = require('./schema/resolvers');
const { typeDefs } = require('./schema/typeDefs');

const graphQL = new ApolloServer({
schema: buildFederatedSchema(
{
typeDefs,
resolvers
}
)
});

const app = express();

app.use('/api', useSofa({
schema: buildFederatedSchema({
typeDefs,
resolvers
})
}));

graphQL.applyMiddleware({ app });

const port = process.env.PORT || 4001;

app.listen({ port }, () => {
console.log('Server ready http://localhost: ' + port);
});
```

But then, when I try to federate that microservice, and any number of microservices, into a gateway API as instantiated below, I recieve a `null` value whenever I make a REST query.
```const express = require('express');
const { useSofa, OpenAPI } = require('sofa-api');
const { ApolloServer } = require('apollo-server-express');
const { ApolloGateway } = require("@apollo/gateway");

const gateway = new ApolloGateway({
serviceList: [
{ name: "salesLineItem", url: '${process.env.NUTRIEN_SALES_DOCUMENT_SERVICE_URL}/graphql' },
{ name: "salesDocument", url: '${process.env.NUTRIEN_SALES_LINE_ITEM_SERVICE_URL}/graphql' },
{ name: "productPrice", url: '${process.env.NUTRIEN_PRODUCT_PRICE_SERVICE_URL}/graphql' },
]
});

(async () => {
const { schema, executor } = await gateway.load();
console.log('Gateway schema', JSON.stringify(schema));
const graphql = new ApolloServer({ schema, executor });

const app = express();

app.use(
'/api',
useSofa({
schema
}),
);

graphql.applyMiddleware({ app });

app.listen({ port: process.env.PORT }, () => {
console.log('Server ready http://localhost:' + ${process.env.PORT});
});
})();
```

So, the same query succeeds when made against the microservice, but returns `null` when made against the federated gateway service. Is it not possible to use SOFA with a federated Apollo server?

Start by reproducing the REST query through the gateway entry point, focusing on ApolloGateway's serviceList and app.use('/api', useSofa({ schema })). Compare that result with the same query against the microservice setup. Done means determining whether federated Apollo schemas can return the expected REST data through SOFA, with a fix or a documented limitation.

Written by the indexing model from the issue text.