(refactor) Replace federation with subgraph

[iMobs]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Replaces @apollo/federation with @apollo/subgraph. The buildFederatedSchema method was deprecated and under the hood, just a reexport of buildSubgraphSchema.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

With this in place, the nest docs on federation should be updated to add @apollo/subgraph instead of @apollo/federation.

I've also added type annotations for the dynamic imports since loadPackage just returns any.

Followup to #1797

[kamilmysliwiec]

This has been address in this PR #1893