feat: Create a Block Content Proof API

[jsync-swirlds]

Persona

As an Hedera Network User

Request

I want to Prove a particular value as of a particular block

Goal

So that I can assure any third party that their transaction was executed correctly

Technical Notes

There are many uses for a Block Content Proof. As part of the State Proof Service (or as a separate Block Content Proof Service) we need to define an API that permits a client to request a Block Content Proof for a particular item in a particular block. This API must define request, successful response, and all probable failure responses (e.g. block does not exist, block not available, item is not present, item has a different value, etc...).

This feature is to define the API and create the API service in the Block Node. The actual creation of Block Content Proofs is a separate feature.

[AlfredoG87]

Here is my suggestion for this API

Request:

1. one of BlockItem or BlockHash
2. BlockNumber

Response:
one of

1. BlockProof --> Set of Bytes (Hashes)
2. ErrorResponse --> BlockNotAvailable or BlockDoesNotExist or BlockItemHashNotExistsInGivenBlock, etc...

Also, either is part of a new Service, BlockProofService (just another suggestion for the name) or maybe a new method on BlockAccessService. Leaning towards a new Service.

Note:

An external party will have the blockProof which is the blockRootHash signed by the network.

The proof needs to provide all the hashes that the user needs in order to compute from the blockItemHash the rootBlockHash, however the Direction of the hashes matter so I would like to recommend for the proof to be a set of MerkleProofElement that is formed by a Hash (Bytes) and Direction (since we only have left or right a boolean should do it, or it can be a Direction Enum.)