Break out a Fetcher class for grabbing external data

[fillerwriter]

Now that we have a couple of provider implementations out, it seems fairly obvious to me that we should break out the code for fetching external data into a separate class and inject that into the provider classes. Bonus points if we can use something already written.

[fillerwriter]

Dropped in a custom class (ExternalURIFetcher) to use for now. We should still look to see if there's anything better we can use as a library.

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