Fix execution engine integration tests with latest geth version (#6996)

#6937


  - Use `ethers-rs` [`Signer`](https://www.gakonst.com/ethers-rs/middleware/signer.html) middleware for local signing and sending raw txs to geth
- ~~Set `totalDifficulty = 0` through `serde` default if the block does not contain a `totalDifficulty` field~~

Author: EllipticPoint