Consider making the Asset Manager API more like the Publishing API

[floehopper]

Currently the Asset Manager API only supports very generic endpoints, e.g. create, show, update, etc.

Given that our longer-term thinking is to move assets into the Publishing Platform/Content Store, it might make sense to have some "richer" API endpoints that somewhat mirror those in the Publishing API.

So rather than using the generic update endpoint to set a redirect_url on an asset, it might be better to have a specific unpublish or withdraw endpoint. I think this would also make it easier to work out what validation rules should be in place on the Asset model.