Load Ethereum signing keys on the Namada relayer

[sug0]

As of #2012, the relayer is able to read Ethereum private keys from an environment variable. Not only is this a somewhat impractical source to read the key from, it's also quite unsafe, since it requires storing a key in the environment, which child processes can inherit and read from.

To improve this system, we may load keys from sources like namadaw, ledger, or aws kms. Moreover, because we're specifying a private key, we can derive the wallet address from the key, potentially removing the need to keep the ethereum-address CLI/SDK arg (see this link).