Add digest config option for SignedCookiesJar

Two commits with small improvements:

* `config.action_dispatch.cookies_digest` config option
* `AS::MessageVerifier` now returns object preconfigured with `config.action_dispatch.cookies_serializer` && `config.action_dispatch.cookies_digest` values.

/cc @chancancode @tamird
Oh right! Yep, that will do, thanks.