Support usage of hardware wallets and other signers in exmple publisher.rs app

[nategraf]

Currently the example app, publisher.rs, only supports a local wallet (e.g. a raw private key provided as an input). It would be good to have the example support hardware wallets or other options to allow folks to try this out on Ethereum Mainnet without transferring funds to an in-memory wallet.

[nategraf]

For this we will implement Ledger as the one additional example wallet