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
Since the MAC is applied outside of the encryption envelope, a slightly better way to test this is to try verifying the message. Something along the lines of...

```ruby
sign_secret = @request.env["action_dispatch.encrypted_signed_cookie_salt"]

sha1_verifier     = ActiveSupport::MessageVerifier.new(sign_secret, serializer: NullSerializer, digest: 'SHA1')
sha256_verifier = ActiveSupport::MessageVerifier.new(sign_secret, serializer: NullSerializer, digest: 'SHA256')

assert_raises(ActiveSupport::MessageVerifier::InvalidMessage) do
  sha1_verifier.verifier(cookies[:foo])
end

assert_nothing_raised do
  sha256_verifier.verifier(cookies[:foo])
end
```

(I know that we generally want to stay away from using `assert_nothing_raised` as it doesn't really do anything, but in this case if the verifiers are not working at all I guess the first assertion would have caught it..? Feel free to propose better alternatives too)