[mpt] Abstract out hash function from Merkle Compute

[kkuehlz]

Introduces a the MerkleHasher trait which is plumbed through as a template parameter for the different Compute implementations. This PR is fully backwards compatible with the current implementation. Every Compute implementation uses the Keccak256Hasher. This PR prepares the compute module for pluggable hash functions, namely to support the coming BSTORE subtrie, which will use blake3.

The trait allows us to reuse all the existing code for merkelization while customizing the hash function used. The blake3 addition will look roughly like the following:

struct Blake3Hasher
{
    static_assert(BLAKE3_OUT_LEN == HASH_SIZE);

    static void hash(unsigned char const *in, size_t len, unsigned char *out)
    {
        blake3_hasher hasher;
        blake3_hasher_init(&hasher);
        blake3_hasher_update(&hasher, in, len);
        blake3_hasher_finalize(&hasher, out, HASH_SIZE);
    }
};

using BlockStorageMerkleCompute =
        MerkleComputeBase<Blake3Hasher, ComputeBlockStorageLeaf>>;

[Copilot]

Pull request overview

This PR introduces a MerkleHasher trait to abstract hash functions from the Merkle Compute implementations, preparing the codebase for pluggable hash functions while maintaining full backwards compatibility. Currently, all implementations continue to use Keccak256, but the abstraction enables future support for alternative hash functions like Blake3 for the upcoming BSTORE subtrie.

• Introduces MerkleHasher C++20 concept and Keccak256Hasher implementation
• Threads the Hasher template parameter through all Compute classes
• Refactors functions from .cpp to header as templates while improving code quality

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
category/mpt/merkle_hasher.hpp	New file defining the MerkleHasher concept, HASH_SIZE constant, and Keccak256Hasher implementation
category/mpt/merkle/node_reference.hpp	Templated to_node_reference function with MerkleHasher parameter and replaced KECCAK256_SIZE with HASH_SIZE
category/mpt/compute.hpp	Added MerkleHasher template parameter to all Compute classes and helper functions; moved previously non-templated functions from .cpp to header as templates; renamed hash methods for generality
category/mpt/compute.cpp	Removed functions that are now templated in the header, keeping only encode_empty_string
category/mpt/test/test_fixtures_base.hpp	Updated test fixtures to explicitly specify Keccak256Hasher template parameter
category/execution/ethereum/db/util.cpp	Updated all Compute type instantiations to include Keccak256Hasher as the first template parameter
category/mpt/CMakeLists.txt	Added merkle_hasher.hpp to the build configuration

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[marcograss]

would it be possible to check second.size() similarly to first.size() with MONAD_ASSERT?

also in theory concat_len could integer overflow although unlikely because the data we are summing sizes is backed by memory, however we can explore options like checked arithmetic