[managed-federation] New plugin: `useManagedFederation` which loads and keeps up to date a GraphOS managed supergraph

[EmrysMyrddin]

Is your feature request related to a problem? Please describe.

GraphOS is offering a managed federation service which automatically generate and manage a supergraph.

Apollo Gateway can load automatically this supergraph. We should provide the same thing with a plugin.

Describe the solution you'd like

A plugin which registers an executable schema based on the managed supergraph.
It should automatically fetch it and maintain it up to date via polling.

Additional context

Apollo's managed federation documentation