Prototype HttpCache as a Client middleware

[adamziel]

Work in progress, do not merge.

Introduces a CacheClient class that wraps Client to cache HTTP responses locally. It supports:

• Request headers: If-None-Match, If-Modified-Since
• Response headers: cache-control, etag, vary, last-modified, expires, max-age, s-maxage
• 304 responses
• Cache invalidation

Remaining work

This PR works, but is left as a draft until someone provides a compelling use-case. Once that happens, we'll need to:

• Merge HttpClient: Separate Client from Transports #19
• Turn CacheClient into CacheMiddleware that sits between RedirectMiddleware and HttpMiddleware