Add support for managed federation

[StarpTech]

Hi, it would be great to support managed federation. The difference between federation is that the gateway is no longer responsible to configure and fetch the service schemas at runtime. The entire composed graph is provided by a single versioned artifact to the gateway.

References

• https://www.apollographql.com/docs/federation/managed-federation/overview/
• https://www.apollographql.com/docs/federation/#supergraph-schema

[dnerdy]

Have you taken a look at the federation example? The datasource poller fetches SDLs from services, but it could just as well read SDLs from a versioned file. That's what we do for our gateway. We write SDLs to a JSON file, and the file includes a version in the filename. When we deploy services, we first roll out the updated services. During the rollout the gateway continues using the current production config. Once services have updated, we notify the gateways there's a new version and they download and load the new config.