Adapt connectors expansion and validation to handle abstract types (interfaces and unions)

[benjamn]

[!Note] This PR builds on #8616 and #8626

Supporting the ... syntax in the JSONSelection mapping language (PR #7957) allowed for conditional selections, which allows for conditionally determining the __typename of an object in GraphQL, which enables the use of abstract types (or type polymorphism): interfaces and union types that might be one of several concrete types, with the exact __typename and associated fields determined at runtime.

This PR updates the rest of the connectors system (beyond the JSONSelection language) to tolerate abstract types: relaxing validation to allow for abstract types, but also validating the fields of each concrete type; rewriting expansion to understand interfaces, unions, and JSONSelection strings that compute their __typename and other fields; and updating the shape crate to support this new functionality. The new expansion and validation traversal logic is now Shape-based, which is the achievement that allows for static analysis of conditional __typename selections.

[apollo-librarian[bot]]

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* graphos/routing/(latest)/performance/caching/response-caching/observability.mdx

Build ID: a5a5cbc4318c25fe0dda6b9a Build Logs: View logs

URL: https://www.apollographql.com/docs/deploy-preview/a5a5cbc4318c25fe0dda6b9a

[benjamn]

Going back to draft status because this PR needs a rework to make better use of ConnectSpec version gating to ensure backwards/forwards compatibility.

[github-actions[bot]]

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

[benjamn]

Update: this PR has gotten a lot simpler (just two commits now!), but I still need to add proper ConnectSpec version gating, so we'll be in Draft until then.