Interpret the state of a pipeline run for the UI, in the API

[squaremo]

The UI needs to know where is the pipeline run "up to":

• which revision is running through the pipeline
• which environments have reached a ready state with the revision
• in each environment, which targets are healthy and which not
  • for targets that are not ready, why are they not ready (to the detail that the application object status allows)
• is a promotion waiting to succeed for some environment
• should it show an approval button for a particular environment

This should probably be done in concert with showing these things in the UI, so we can be sure it has all the things needed. NB some items can be aggregated in the WGE API, or calculated by the UI itself. But, for integration with other systems (e.g., Backstage plugins), we should try to make the pipeline object status useful on its own.

[squaremo]

Looking at the relevant mockups in https://www.figma.com/file/IVHnM9iyeFWpd11evtY8ux/Weave-GitOps?type=design&node-id=20414-72393&mode=design&t=TIcx53hK0tYP3iSH-4, I don't think there's any summarising needed after the promotion status is added in weaveworks/pipeline-controller#197. Closing this for now; we may want to revisit when we go back to augmenting the GUI view.