This commit adds support for `:message_pack` and `:message_pack_allow_marshal`
as serializers for `config.action_dispatch.cookies_serializer`, just
like `config.active_support.message_serializer`.

The `:message_pack` serializer can fall back to deserializing with
`AS::JSON`, and the `:message_pack_allow_marshal` serializer can fall
back to deserializing with `AS::JSON` or `Marshal`.  Additionally, the
`:marshal`, `:json`, and `:hybrid` / `:json_allow_marshal` serializers
can now fall back to deserializing with `AS::MessagePack`.  These
behaviors make it easier to migrate between cookies serializers.