Store endpoint: get_tx_inputs

[Dominik1999]

@plafer to create

[plafer]

Implement a gRPC endpoint which corresponds to this method. That is, inputs a ProvenTransaction, and outputs the protobuf version of TxInputs.

[bobbinth]

To specify the endpoint in terms of request/response - here is what I'm thinking:

Request {
    accounts: [list of account IDs involved in the transaction],
    nullifiers: [list of nullifiers created in the course of the transaction],
}

Response {
    accounts: [list of account state hashes for each of the specified IDs],
    nullifiers: [a list of boolean values for each nullifier indicating whether the nullifier has been consumed],
}

[bobbinth]

One potential modification to the above: maybe instead of boolean values for the nullifiers we should return a list of block heights at which the nullifiers were inserted into the chain (or 0 for nullifiers which haven't been inserted). This will help with better error messaging as we'll be able to say that a given nullifier was already consumed at such and such block.

[hackaugusto]

@bobbinth I believe this can be closed, right?

[hackaugusto]

implemented by #60