API Server - Add "Repo Charts Fetcher" interface

[jackfrancis]

We're currently relying on a black box implementation inside the data.Charts.Refresh method. For the data.cache.cachedCharts implementation of data.Charts specifically, we have an http.Get implementation that operates against the URLs in the knownRepos instance slice map.

Less specifically, what we want is to be able to pass an interface into Refresh so that we can provide a fetcher implementation that is backed by mock data, for tests. Until then we're relying on actual HTTP connections to the prod repos to test data.cache.