Take Headers and Cookies into consideration when creating cache key

Magnolia doesn't support caching for different headers or cookies, thus making it impossible to cache responses with different cookie or header traits.

This ticket aims to improve that functionality, so that it's possible to specify for which headers (or cookies) the response should be cached.

Cache should know (either via configuration or by plug-in code that resolves the headers, for example from TraitRegistry) what headers are subject of change and cache based on that information.