validator pre-genesis wallet - add Tendermint keys format

[tzemanovic]

Add tendermint's format for validator's consensus and node keys in the validator pre-genesis wallet to make it easier to use these in places where TM format is expected (e.g. p2p config). We can still keep the PKs in our format (ad-hoc hex currently and bech32m to be added from #849) if we add the TM format as new fields (subject to some runtime validation that they match)