Fix value comparison, argument merging, and link feature addition/extraction in composition

[sachindshinde]

While reviewing recent code changes that released in 2.9.0, I noticed a few bugs in the code, so I've pushed some fixes for them in this PR (along with some fixes for more ancient bugs).

I'll comment on the code with more details, but to summarize the fixes:

• Value comparison/default value application has been updated to handle nulls/undefineds better.
• Composition argument strategy code has been refactored to handle nullability more cleanly (and fix some bugs around nullability).
  • Note I've commented out the unused SUM strategy, as using it currently wouldn't yield expected results since directive applications are deduped before being merged.
• The logic that applies @link applications to the supergraph schema for supergraph specs needed by subgraph directives has been updated to handle specs that have multiple directives that need composing (the behavior before was adding spurious directive definitions to the supergraph schema).
• Some ancient logic around computing directive/type names for @core/@link was off.

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🦋 Changeset detected

Latest commit: 2e54f9873821ebc144be129bb0c816f356b1f142

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 7 packages

| Name | Type | | --------------------------------------- | ----- | | @apollo/federation-internals | Patch | | @apollo/gateway | Patch | | @apollo/composition | Patch | | @apollo/query-planner | Patch | | @apollo/query-graphs | Patch | | @apollo/subgraph | Patch | | apollo-federation-integration-testsuite | Patch |

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