* refactor auths broadcast cmd in alignment with #6216
* add TxResponse proto definition, and related refactoring
* re-run make proto-gen, updating staking.pb.go
* cleanup TxResponse tests to handle nil return values
* properly handle nil Tx value in TxResponse's implementation of UnpackInterfaces
Co-authored-by: Federico Kunze <31522760+fedekunze@users.noreply.github.com>