Submit Ethereum transaction integration test

[HCastano]

The SDK team is currently trying to use Entropy to produce a signature that they can use
to submit a transaction to Ethereum.

We should have a similar check on our end in order to ensure that the Core code works as
expected without the mystery that is Javascript.

While I put this as an integration test in the issue title, we can also just have this as
a small script for now to at least prove that it works.

Example flow would be:

• Craft an Ethereum transaction which performs a balance transfer
• Submit a signature request to Entropy using this unsigned transaction as the message
• Submit signed transaction to an Ethereum testnet

A useful library for this would be ethers-rs.

[HCastano]

Actually submitting to an ETH testnet makes things kinda tricky and we don't really want to have this as part of our CI. Just setting up the transaction as was done in #664 is enough.