Support more options in cache-control extension

Totally agree that it would be great to add some more features. Curious which you are interested in and how you might see them play out in the extensions?

@jakubriedl wrote in apollographql/apollo-cache-control-js/#18

> At this moment caching supports just `maxAge` and `scope` hints. it is a good start but there are much more features which should be supported. In general, it would be great if all applicable HTTP caching features would be supported.
>
> To mention few which we need at this moment
>
> - [Age](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Age) if the responded resource is not absolutely new we can respond the current age. So the cache time can be calculated as `maxAge - Age` and so we can prevent doubling the cache time.
> - [Vary](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Vary) we are using standard content negotiation for determining client language. This header allows to tell the engine what headers should be included in cache key to determine the correct response.
>
> Also, it might be useful to integrate match and time-based validation (`ETag`, `If-None-Match`, `If-Modified-Since`) with the query using directives to reduce the amount of transferred data.
>
> As a final note, I will mention that we are willing and able to help with design, development, and testing but you already know that 😃

Start by reviewing the cache-control extension and the referenced apollographql/apollo-cache-control-js/#18 discussion. Define which HTTP caching options, including Age, Vary, ETag, If-None-Match, and If-Modified-Since, should be supported and how query directives would expose them; done requires an agreed design and corresponding implementation and tests.

Written by the indexing model from the issue text.