DFP: Implement view handler for custom rendering

[mxhCodes]

According to https://support.google.com/dfp_premium/answer/6366910?hl=en native ads are always being rendered as "SafeFrames" by default for web apps, which are iframes in the end. This is basically what the "default HTML view" currently does.

For custom rendering, a new view handler would be needed.

I'm curious though how we'd accomplish the custom rendering to be commonly useful. It looks like a customized use case and its implementation seems to vary on the project's needs. Thoughts?