pre-rebase-deposit_contract: fe: add ETH 2.0 deposit contract port + differential tests

Port the official mainnet deposit contract (0x00000000219ab540356cBB839Cbe05303d7705Fa) to Fe and verify byte-for-byte equivalence against the Solidity reference. Each test vector: Pre-computes the expected deposit_data_root in Rust so both contracts accept the deposit. Calls deposit(...) on the Fe and Solidity contracts with matching value. Asserts get_deposit_root() / get_deposit_count() agree…