feat(x/data): add simple privacy preserving merkle tree algorithm

[aaronc]

Description

This PR defines a merkle tree algorithm for anchoring and signing graph data in a provable, privacy-preserving manner. Anchored data can provably contain any sub-graph of the anchored graph so graphs can be shared which omit personally identifiable information such as geo-locations, personal names, etc.

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[aaronc]

We could add hash iterations in these steps to make this more resistant to brute forcing. The number of iterations could either be fixed or configurable, although if it is configurable the question is where this is stored? One option is to create a few variants of this enum such as GRAPH_MERKLE_TREE_SIMPLE_PRIVACY_PRESERVING (no iterations), GRAPH_MERKLE_TREE_SIMPLE_PRIVACY_PRESERVING_10000, GRAPH_MERKLE_TREE_SIMPLE_PRIVACY_PRESERVING_100000, etc. Even a high iteration count will not protect triples with a small value space (ex. booleans), but could better hide LAT/LON location data for instance.