Implementation of Nimbus eth_getProof RPC Endpoint.

[bhartnett]

This PR includes changes to implement the eth_getProof RPC endpoint implemented as follows:

• The account and storage proofs contain an array of RLP encoded merkle patrica trie nodes starting from the root node, following the path of the trie key until reaching the leaf node. The leaf node is also included as the final node in the proof.
• When getting an account that doesn't exist in the state we return values set with defaults. Balance, nonce, codeHash and storageHash will be zero in this case.
• When returning an account that doesn't exist in the state we also return a proof that proves the absence of the value in the state. In this case the proof will contain all nodes that do exist following the path of the provided key.
• If an existing account is not a contract account then the code hash will be equal to the keccak256 hash of an empty string.
• If an account has no storage slots populated then it will have a storageRoot equal to the keccak256 hash of the empty root node.
• If the slots array parameter is empty then we return an empty list for the storageProof.
• If we supply a non empty array of slots as a parameter then we return a storage proof for each non existent slot with the slot value equal to zero. These proofs will prove that the slot doesn't exist in the state. If the storage state is empty then the proof will be empty. If the storage state is not empty then the proof will contain all rlp nodes following the path of the slot key.

Here is the link to the spec: https://eips.ethereum.org/EIPS/eip-1186. Note that the spec is in the 'Stagnant' state so it was never moved to final.

[bhartnett]

Is this the correct way to update the copyright notice? Or should it be 2023-2024?

[jangko]

2023-2024

[bhartnett]

Thanks, I'll update in that case.

[bhartnett]

Exporting the new proof procs to be used with the ReadOnlyStateDB type.

[bhartnett]

This code was copied from the HexaryTrie library in order to implement getBranch on the CoreDbPhkRef type.

[bhartnett]

The getBranch code returns a single node with a value of emptyRlp (zero encoded as rlp) if the trie state is empty. We remove this because this isn't a valid proof and causes the proof verification to fail. Also Geth returns an empty list in this case so we do the same.

[bhartnett]

We return zero values (default values) for the account if the account doesn't exist, including the codeHash and storageHash.

[jangko]

looks good to me

[bhartnett]

looks good to me

Great. Planning to merge it after I update the copyright notices.