Support caching extension data

Feature request for `apollo-server-plugin-response-cache` to support caching extension data.

**Use case:**
We are using extension data to power AB experiments. Rather than adding AB test metadata to our schema we are just returning it in extension data.

Currently this is incompatible with `apollo-server-plugin-response-cache` because it it disgards extension data before saving to the cache.

**Could this be a plugin?**
It's a feature request for the existing plugin `apollo-server-plugin-response-cache`

**Is there a workaround?**
Forking the plugin `apollo-server-plugin-response-cache` and extending it to cache and restore `extensions` along side `data`.

Start by inspecting the apollo-server-plugin-response-cache implementation and its cache serialization and restoration flow. Verify how response data is currently stored, then make sure extension data is preserved alongside it and confirm cached responses restore both data and extensions.

Written by the indexing model from the issue text.