feat: allow persistence of optimistic data

[jackstudd]

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This change allows to pass a includeOptimistic boolean to the CachePersistor options. This boolean is further passed down to Apollo's cache.extract() function, which accepts a boolean, to allow the extraction (and thus, persistence) of optimistic data.

This PR aims to solve issues related to implementing offline use of applications relying on Apollo Client like https://github.com/apollographql/apollo-cache-persist/issues/410

[apollo-cla]

@jackstudd: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/