Implement webhooks to trigger imago remotely via http

[philpep]

For example:

$ curl -X POST https://imago.example.com/deployment/app
{
   "containers": {
        "app": {
           "new": "registry/app@sha256:YYYY",
           "old": "registry/app@sha256:XXXX"
    },
    "initContainers": {}
}

The webhook url could be protected by basic auth / ingress.