Cookies digest config option (pt. 2)

Another take on https://github.com/rails/rails/pull/16434

Now with comma, support for encrypted jar plus better test for verifier and new ugly test encryptor.

/cc @tamird @chancancode
Is there a specific reason for this change? It seemed a little bit weird to me – the name of the option clearly implies that it should only affect how cookies are serialized, whereas the application verifier is more general purpose.

If we want this to be customizable (I guess why not), it seems better to add an `options` parameter to `Rails.application.message_verifier` and pass it down to `AS::MV`. wdyt?