For adzerk integration, we need to ensure we can swap out spoc urls from the current one to the new one.

Two places we need to do this.

1. in the layout response, which is not a client side change.
2. in the hard coded layout.

Depending on our rollout strat, we may also need to create something so we can toggle between two versions, probably changing the layout in the Discovery Stream pref, or setting up something in shield.

For the hard coded layout, when we're ready to turn this on, we just need to update the spoc endpoint.

We can do that whenever we're ready, but at the latest, we can do it during 70 and uplift to 69. The change is about as complex as a pref change.