Write document responses as struct type instead of returning string

[naomijub]

Since the introduction of speculative tx, #35, there is the necessity to deal with edn documents

gXTDB/resources/transactions.proto

Line 63 in f26fb9b

string edn_document = 6;

which currently are dealt as strings and left for the rust side to handle. If we could parse this as a protobuf.google.Value for client to use it as a serde_json::Value