experimental: MPT block witness for Erigon 3

[antonis19]

Experimental eth_getWitness RPC call implementation that computes the block witness for the MPT.

This works roughly along the following lines:

• unwind execution to the start of given blockNr
• execute the block ephemerally and record touched account and storage keys using a custom StateWriter
• Load each of these keys from the state trie at blockNr-1 and from the resulting grid create a merkle path for each of these keys
• Merge all the merkle paths to obtain the witness trie
• Verification step: execute the block "statelessly" by using the witness trie as state and check that the resulting state root matches the state root in the header of the block
• return the serialized witness

This implementation is currently incomplete and still lacks support for the following cases which will be future work:

• witness for keys not in the current state trie (often resulting in wrong witness trie root)
• edge cases where extension, account and storage are fused into one cell in the grid (needs "de-fusing" into multiple expanded trie nodes)
• unit tests

The code currently works correctly in 85% of the blocks of a small test chain of 161 blocks, for the unsupported cases mentioned above the RPC method will throw a root hash mismatch error.

I am proposing to merge the current PR even though the implementation is incomplete, so that we can incrementally add these features, and to avoid arduous merges or rebasing with main.

[awskii]

@antonis19 do we need https://github.com/erigontech/erigon/pull/12783 then?

[antonis19]

@awskii nope, just closed that PR.