Add support for cache-control headers (including ETag)

[jacksontj]

As commented in #41 caching can likely be easier to achieve with simple cache-control headers. To do this in promxy we want 2 options:

(1) TTL for Cache-Control headers -- to control how long something is cached
This should be as simple as adding a caching middleware to the API surface (maybe configurable for which paths are cache-able.

(2) ETag headers -- to allow for If-Modified-Since requests.
This will require:

• middleware to fetch the ETag value from proxystorage
• ETag calculation based on servergroup state (which targets, ideally "health" of those targets)

[jacksontj]

Now that other caches (e.g. trickster) support E-tag (trickstercache/trickster#234) this could be interesting again. I did have a hacked up version which seemed to work -- just need to get some time to clean that up and do all the requisite testing.