feat(besu): private transaction support

Depends on #367

Is your feature request related to a problem? Please describe.

Feature request: I need to be able to issue private transactions to a Besu ledger via the Cactus API client.

Describe the solution you’d like

1. Either a separate endpoint next to the current POST /run-transaction one or as an extension of it. Not yet sure which one is more feasible/makes more sense.
2. Test coverage for the new feature
3. Private transactions must not be logged by Cactus for obvious reasons.

Describe alternatives you’ve considered

Nothing. There is no doubt in my mind that we need this feature.

Additional context

Here’s the Besu documentation regarding private transactions: https://besu.hyperledger.org/en/stable/Concepts/Privacy/Private-Transactions/

cc: @takeutak @sfuji822 @jonathan-m-hamilton

[hartm]

@petermetz

I guess what I’m saying is this is where we will want formal guarantees of privacy. We can ask some of the academics like Rafael and Benedikt to help us with this.

[RafaelAPB]

From an implementation perspective: I was trying to implement the functionality of issuing private transactions on the connector, but I faced some issues. I talked to a Besu’s maintainer some time ago: https://github.com/hyperledger/besu/issues/504, and it looks like that " private equivalent of https://web3js.readthedocs.io/en/v1.2.0/web3-eth-contract.html hasn’t been implemented yet", so probably some workarounds are needed.

I’ll answer on #367 regarding design