Create Task to update GitHub PR status

[bobcatfish]

Expected Behavior

We should be testing and releasing Pipelines with Pipelines.

To do this, we need a Task that knows how to update a GitHub PR with the results of a check.

• Update a PR with the result of the "check", i.e.:

The Task

• Inputs:
  • git repo (maybe? this is a github thing tho)
  • some kind of new resource(s) that contains:
    • github endpoint details
      • PR to update the checks of
      • The check to update
      • What status to update
      • Link to details

Will need to look more into what needs to be specified to update a check, possibly investigate prow's implementation

Actual Behavior

prow does this for us, not sure how.

Additional Info

• Actually triggering this is not in scope for this issue, it's fine to test manually (we'll be linking these together in a later issue)
• We're going to need a service account that can update GitHub PR checks
• This can be considered a POC for "notifications" (Design: Notifications #49) (+ @dlorenc )

[dlorenc]

Ref #778

[bobcatfish]

• These days dogfooding is in full swing and most issues are in https://github.com/tektoncd/plumbing
• This is the project we are using to track our current dogfooding efforts: https://github.com/orgs/tektoncd/projects/1
• This document contains our dogfooding plans