fix: support function expressions in config

[miii]

This PR adds support for use of function expressions inside Apollo config, e.g. to define custom type policies.
https://www.apollographql.com/docs/react/caching/cache-field-behavior

The current serializer, JSON.stringify, simply removes all function expressions as it cannot serialize them.
This is also mentioned in #443.

A common use case for this is offset-limit paginated queries where new entries may be merged with previously fetched results (instead of treating them as different queries).
https://www.apollographql.com/docs/react/pagination/offset-based#the-offsetlimitpagination-helper

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