Add ethrex client to Hive

[mpaulucci]

We currently have a fork of hive: https://github.com/lambdaclass/hive/tree/master

In this fork we have our client setup: https://github.com/lambdaclass/hive/tree/master/clients/ethrex

The goal of this ticket is to add ethrex to the official hive repository: https://github.com/ethereum/hive

To be able to do that, first review the code and ensure that we're passing all options correctly (a good idea is to compare with other clients).

There have been some changes in Hive outside of the client/ethrex code to make more tests pass. These are out of scope of this ticket.

When the PR to the official Hive repo is merged, we should sync our forked repo.

[fedacking]

Internal pull request to view changes fedacking/hive#1