This commit adds support for `:message_pack` as a serializer for
`MessageEncryptor` and `MessageVerifier`, and, consequently, as an
option for `config.active_support.message_serializer`.
The `:message_pack` serializer is implemented via
`ActiveSupport::Messages::SerializerWithFallback` and can fall back to
deserializing with `AS::JSON`. Additionally, the `:marshal`, `:json`,
and `:json_allow_marshal` serializers can now fall back to deserializing
with `AS::MessagePack`.
This commit also adds support for `:message_pack_allow_marshal` as a
serializer, which can fall back to deserializing with `Marshal` as well
as `AS::JSON`.
af6d83521c