Add middleware feature

[MoskitoHero]

Barley would be much more versatile and extensible if there was some kind of middleware feature.

The dependency to Dry-Types would be moved to a middleware, for instance, which would make a lot of sense and make Barley leaner in the Gemfile by default.

Other candidates for the middleware would be caching and context - all the stuff that is not just focused on serializing to a hash.

This would also guarantee that Barley remains very stable and fast in its core version, all extra features being opt-in.