Add support for `Vary` response header in cache resolution

### Feature Summary

Harper can handle `Response`s in cache resolution, facilitating general HTTP caching. However, there is no support for storing separate resource representations based headers listed in the `Vary` response header.

### Proposed Solution

The caching table will use an `alternates` field which can contain an array of alternate representations of the resource. When a `Vary` response header is received, the full key should be calculated from the indicated request headers, and the resolved body/data should be place in the alternates array if it is a new, or replace an existing entry if there is a matching one.
The request handling in Harper should also be able to detect the presence of alternates in the cache, and select the correct one.