feat: replace query instance with query observer

[boschni]

This MR addresses a few things:

• Replace QueryInstance with QueryObserver. The purpose of this entity is to observe a query while having the ability to switch to different queries.
• Add keepPreviousData query option to keep any previous data while loading new data when the query key changes. Would it be an idea to deprecate the resolvedData and latestData properties?
• Make sure a query is refetched on windows focus when any observer indicates if it should be refetched.
• Make sure when a user calls a refetch/clear/fetchMore hook callback, the configuration from that specific hook is used.
• Some preparation work to allow configuration to be a single object.

[vercel]

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[tannerlinsley]

🎉 This PR is included in version 2.7.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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