Write to content API

This PR adds writing to content API for when there are updated to live.

Nothing too exciting, it just writes in the agreed format to the specified endpoint. It is behind a switch and is initially off.
`ConfigAgent` had to be moved to `common` as now `facia-tool` and `facia` need it. So unfortunately I had to name this `ConfigAgentTrait` to get it away from its companionship. It could probably have a better name.

There are two new metrics for `failure` and `success`.

As of writing, there are two outstanding issues upon writing to the content api:
 - `Collection title` is still not optional
 - `backfill` now needs to be optional