refactor: define configuration merging strategy

[boschni]

The current config merging looked a bit confusing so I tried to simplify this a bit and to define a formal strategy:

When using hooks the config will be merged in the following order:

1. Defaults.
2. Defaults from the hook query cache.
3. Combined defaults from any config providers in the tree.
4. Query/mutation config provided to the hook.

When using a query cache directly the config will be merged in the following order:

1. Defaults.
2. Defaults from the query cache.
3. Query/mutation config provided to the query cache method.

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[tannerlinsley]

Ready?

[boschni]

Yes this one is ready to be merged

[tannerlinsley]

🎉 This PR is included in version 2.7.1 🎉

The release is available on:

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

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