Federation 2 updates about apollo-federation-subgraph-compatibility

[hwillson]

@prasek a few quick notes about external dependencies this project relies on that aren't Federation 2 ready:

1. Apollo Gateway Docker Container doesn't support Fed 2.

This project relies on https://github.com/apollosolutions/gateway-container, which doesn't have a Fed 2 compliant version available yet. For now I've cloned / updated the project to continue with Fed 2 work, but I'm assuming we'll want to update the gateway-container project upstream (which I'm happy to do but I'm guessing we're waiting until Fed 2 GA - cc @lennyburdette).

2. The latest available version of @apollo/rover on npm doesn't support rover fed2 commands out of the box.

This project is setup to install and use rover from npm, but the rover fed2 commands are not usable unless the Federation 2 rover plugin is installed separately (which unless I'm missing something isn't installable via npm). I'm not sure if there are plans to make this installable via npm or if this functionality will just become part of the rover core after launch. Not a blocker since rover can of course be installed outside of npm, but we'll need to make changes to this project if the fed2 commands won't be available via npm without installing the fed2 plugin separately (cc @EverlastingBugstopper).