fix(response caching): respect interface objects as entities (#8582)

Co-authored-by: Renée <[email protected]>
Co-authored-by: Duckki Oe <[email protected]>

Author: 3 people

.changesets/fix_response_caching_treat_interface_objects_as_entities.md

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+### Fixes response caching by treating interface objects as entities
+
+Interface objects can be entities but response caching wasn't treating them that way. This fix makes sure they're respected as entities so that they can be used as cache keys.
+
+By [@aaronArinder](https://github.com/aaronArinder) in https://github.com/apollographql/router/pull/8582