Support native output to Atom Syndication Format

[justinquinnb]

https://www.ietf.org/rfc/rfc4287.txt

[justinquinnb]

https://validator.w3.org/feed/docs/rfc4287.html

[justinquinnb]

Under the hood, this should just map a complete aggregation output into an ASF-compatible response. Endpoints should include a "custom" edge as well as a "preset" one.

The presets are configured the same way as any other preset aggregation. In other words, defining a preset aggregation makes it accessible via its ID and the traditional /aggregation/preset/{id} endpoint and the aggregation/atom/preset/{id} endpoints.

[justinquinnb]

https://validator.w3.org/feed/docs/atom.html