exchanges/graphcache: add rootFields option

[ckknight]

Summary

Fixes #3821

Set of changes

Non-breaking change to @urql/exchange-graphcache:

• Adds a new, optional property to CacheExchangeOpts called rootFields that allows specifying these name overrides
• Updates Store to observe the new rootFields property, allowing it to still be overridden by schema (if provided).

[changeset-bot]

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Latest commit: d042fc1

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