Add ethrex as an EL

# 🎯 Problem to be solved

We support 2 ELs in this repo so far. 4 ELs could give great redundancy to any particular one failing/halting/forking. We should consider adding https://github.com/lambdaclass/ethrex as an option. 

# 🧪 Tests

- [ ] Works in local docker-compose
- [ ] Has a attested on a testnet at least once

# 👐 Additional acceptance criteria



# ❌ Out of Scope


