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

https://github.com/sigp/lighthouse/issues/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~~

testing/execution_engine_integration/Cargo.toml

@@ -7,7 +7,9 @@ edition = { workspace = true }
 async-channel = { workspace = true }
 deposit_contract = { workspace = true }
 ethers-core = { workspace = true }
+ethers-middleware = { workspace = true }
 ethers-providers = { workspace = true }
+ethers-signers = { workspace = true }
 execution_layer = { workspace = true }
 fork_choice = { workspace = true }
 futures = { workspace = true }