metadata-service: Add new datasource to download metadata

WIP

Thoughts on this approach? I'm recursively requesting all of the metadata endpoints and building a meta_data.json blob to return. In my testing, it took tens of milliseconds to generate. When I used pkg.HttpClient, it took about half a second to generate.
I retested this and saw much different results. I don't see a significant slowdown anymore. I'll just use the pkg.HttpClient.