feat: use apollo-rs to auto-use Federation 2

[EverlastingBugstopper]

when any subgraph contains an @link directive, rover should just use federation 2 composition under the hood.

right now the only way to use federation 2 is to set federation_version: 2 in supergraph.yaml

this means if you pass a bunch of subgraphs with @link directives, rover supergraph compose will use federation 1 composition by default. we should instead detect the existence of an @link directive, and use SupergraphConfig::set_federation_version to FederationVersion::LatestFedTwo here.

@lrlna has kindly promised to pair with me on this next week - apollo-rs should be a big help here.

[EverlastingBugstopper]

for more context see #1089

[prasek]

@EverlastingBugstopper awesome this is great.

will it work like this?

• if federation_version included in supergraph.yaml then always use that version for composition
• if federation_version omitted in supergraph.yaml, then auto-detect federation_version, looking for @link fed2

[EverlastingBugstopper]

yup!