Continue refactoring of PrivValidator started here: #637

TODO: write a proper ADR for this. Some things to think about:

1. move internal fields into substructs, eg. ID and LastSignedInfo
2. change interface to be Address/PubKey instead of GetAddress/GetPubKey
3. move signBytesHRS to be LastSignedInfo.SignBytesHRS
4. generalize persistence so not stuck with a file