Description

Implement map tree APIs with UnhydratedFlexTreeNode directly instead of a wrapping a MapTree.

Store contextual default providers directly in a strongly typed way in the tree. Implicitly generate them using a global context if requested while unhydrated instead of special casing identifiers in the read code paths in multiple places.

Provide contextual defaults as part of prepare for hydration pass instead of its own pass.

Overall, this refactor should make the handling un unhydrated data simpler and more robust:

• It is no longer possible for the unhydrated tree and map tree and/or the maps associating them to be out of sync.
• The special case handling of identifiers has been implemented in a general way that will handle any contextual defaults, and no longer needs special logic in any code path reading from the tree.
• There is no longer questionable ownership of ExclusiveMapTree nodes due to cache lookup.
• Memory use should be slightly reduced by avoiding having the separate map tree objects and the caches to associate them with the unhydrated nodes.

Breaking Changes

Defaulted Identifiers show up when enumerating fields on an unhydrated node.

This seems like an improvement/simplification (makes unhydrated and hydrated nodes more similar, and identifiers less special), but could theoretically break something.

Reviewer Guidance

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