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

proto/regen/data/v1/types.proto

@@ -141,6 +141,25 @@ enum GraphMerkleTree {
 
   // unspecified and valid
   GRAPH_MERKLE_TREE_NONE_UNSPECIFIED = 0;
+
+  // specifies that the content hash for the graph is based on the following merkle tree algorithm:
+  // 1. the graph is canonicalized using the specified canonicalization algorithm
+  // 2. each triple in the canonicalized graph is hashed as follows:
+  //    a. the number of triples in the graph is used as the subject and predicate salt prefix
+  //    b. the subject is hashed using the specified digest algorithm and the salt prefix
+  //    c. the predicate is hashed using the specified digest algorithm and the salt prefix
+  //    d. the object is hashed using the specified digest algorithm and a salt prefix consisting of the subject hash and predicate hash
+  //    e. the resulting hashes are concatenated
+  // 3. each concatenated triple hash is concatenated with the following concatenated triple hash and these
+  //    concatenated hashes are hashed using the specified digest algorithm and inserted into an array. If
+  //    there is an odd number of concatenated triple hashes, the last concatenated triple hash is hashed
+  //    with itself and placed in the array.
+  // 4. this process is repeated on the resulting array until there is only one hash remaining which is
+  //    the graph hash
+  //
+  // This algorithm is allows for selectively disclosing any individual triples or parts of triples in the graph
+  // without disclosing the entire graph.
+  GRAPH_MERKLE_TREE_SIMPLE_PRIVACY_PRESERVING = 1;
 }
 
 // ContentHashes contains list of content ContentHash.