Implement the /v3/apps/{guid}/manifest endpoint

[danail-branekov]

Background

The CF API /v3/apps/{guid}/manifest endpoint generates a manifest describing the current state of the app. We want to support it.

Acceptance

GIVEN I have pushed an app
WHEN I curl the endpoint via cf curl /v3/apps/:guid/manifest
THEN I get a response as follows:

HTTP/1.1 200 OK
Content-Type: application/x-yaml

---
applications:
- name: my-app
  docker:
    image: some-image
  ...

Dev notes

• We need to return the name and docker.image attributes as a minimum
• Given that we have a state collector implementation already we should attempt to return as full manifest as possible
• Pay attention that the response content type is application/x-yaml, therefore the response body should be yaml rather than json
• Spike commit: 88e32a0