Accelerated hashing using custom RISC-V instructions

## Context The guest machine performs numerous Keccak-256 operations during advances that generates outputs, which requires computing merkle proofs. Currently, `machine-guest-tools` uses a native Keccak-256 implementation that runs through emulation, resulting in significantly higher CPU cycle consumption compared to host-side execution. ## Possible solutions Implement a custom RISC-V instruction for Keccak-256 hashing (with potential SHA-256 support in the future) to offload computation to the host. ## Technical considerations - **Opcode allocation**: Which custom/reserved opcode space can we use without conflicting with future RISC-V specifications? - **Memory handling**: How should the instruction handle virtual-to-physical address translation for large data buffers spanning multiple pages that may trigger page faults? - **Instruction granularity**: Should this be a single instruction or split into multiple operations (e.g., init/update/finalize)? - **Scope**: Should we leverage Solidity-specific hashing optimizations? Should SHA-256 be included? - **Extensibility**: How can we design this to support adding more custom accelerated instructions in the future?