Extract extensions with ssrExchange.extractData

[dios-david]

Hey, I'm using cacheExchange and ssrExchange to be able to use SSR and hydrate in the browser.
I noticed that the optional extensions field from the operation response is not serialised when calling .extractData() on an ssrExchange instance.

Is it possible to make extensions part of the cache data, and serialise that as well?

[dios-david]

Done in #1985