fix(resolvers/federation): only transform federated parent types when they contain @external directive

[jakeblaxon]

This PR seeks to fix the issue raised in #4493

This updates the transformParentType function to only transform parent types if one of their fields contains an @external directive, or if the field is named __resolveReference. This essentially translates to whether the type extends a type defined in a remote schema, or whether the type is a base type.

The objective is to target only the resolvers that the gateway will call from the _entities query in a federated schema. The gateway only guarantees that fields from the @key or @requires directives will be supplied in the parent type, so we limit the parent type to only contain these fields. If the parent type is a base type, however, then its field resolvers (other than __resolveReference) will be called from elsewhere, so we therefore go with the default behavior which is to assume all fields will be available.

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[dotansimha]

Thank you @jakeblaxon , this is great! Can you please add a changeset? Using yarn changeset, and bump the resolvers package with a patch.

[dotansimha]

Never mind, merged, because I want to make sure it will be in the next release ;)
I will add the changeset :)
Thanks @jakeblaxon !