Create a Signer Interface

[joaop21]

A Signer represents an account on the Ethereum Blockchain, which can be used to sign messages and transactions and send signed transactions to the Ethereum Network to execute state-changing operations. It is most often backed by a private key represented by a mnemonic or residing on a Hardware Wallet.

[joaop21]

Some examples of what is already done by other tools:
https://github.com/izelnakri/eth/blob/dd3ed7a34772865891c640e2000532c7d534ae1f/test/eth/transaction_test.exs#L55-L82
https://github.com/izelnakri/eth/blob/dd3ed7a34772865891c640e2000532c7d534ae1f/test/eth/transaction/signer_test.exs#L95-L112

[joaop21]

Another note is that the main goal of this key is to be a wrapper around Ethereumex. We should probably refrain from engaging in this direction of signing transactions and that stuff. We should make the methods available, and that's it.