[Data masking] Ensure `@unmask` can be used with the fragment registry

[jerelmiller]

Fixes #12028

Ensures that fragments registered with the fragment registry can be looked up by the data masking algorithm.

This change adds a new lookupFragment API in the base cache class that allows custom caches to provide lookups for fragments by name. For example, InMemoryCache has the fragment registry, so it overrides the base implementation and uses the fragment registry to lookup a fragment by name.

As a part of this, I've moved the implementation of maskFragment and maskOperation over to QueryManager where I believe it fits better. This makes it a much more private API and makes it much more obvious that custom cache implementations don't need to interact with these APIs directly since we want the core Apollo Client controlling the implementation.

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Latest commit: f6466d84d2adf2115226e48d5d675816e9def987

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