Implement logic for collecting and merging in imported specs

[tninesling]

This change builds on #7875 to merge specs imported by subgraphs into the resulting supergraph. The main functions are collect_core_directives_to_compose (JS source) and validate_and_maybe_add_specs (JS source).

The former iterates over the subgraphs and creates a list of specs to merge in, along with their composition specifications. The output is then passed into the latter to actually apply the specs to the supergraph. The combination of these functions happens in prepare_supergraph which will be implemented in a future ticket.

This also includes a Rust implementation of the JS applyFeatureToSchema.

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[github-actions]

@tninesling, please consider creating a changeset entry in /.changesets/. These instructions describe the process and tooling.

[clenfest]

Sachin had some code where we would return from the function if there was no compositionSpec, but I don't really see how that would be even in the old code, so I guess it's ok.

[tninesling]

There's a section down below where we skip the current link if there's no name in the supergraph. It matches the JS implementation fairly faithfully, but we can always come back and change it later