Test multiple signer versions in the e2e test

[jpraynaud]

Why

We want to test backward compatibility of signers in the end to end test and be able to see if 2 versions of the signer can work with an aggregator.

Important

A previous branch attempted to implement this feature: https://github.com/input-output-hk/mithril/tree/djo/673/e2e_with_two_signer_bin

What

Make the end to end test capable of starting with a signer running a separate binary of a previous version of the signer.

How

• Adapt e2e test to support an optional signer version for one of the signers