add more flexible triggering configuration

[jstrachan]

how that go-scm can handle deployments and a bunch of other webhooks (e.g. when a label is added to a PR, a deployment has been recorded, when a PR becomes unmergeable etc) it might be nice to add a GitHub Actions style configuration to trigger a pipeline using a number of filters. So in addition to the presubmits and postsubmits we could add something like...

presubmits: []
postsubmits: []

# new section for handling events using a more flexible syntax
on:
# list the events and predicates
- # all the events that trigger the pipeline
  # only needs to be 1 event but added multiple event types to show how we could trigger on multiple events 
  release: {}
  label: 
     types: [created]
     name: ok-to-test 
   deployment: {}
   issueComment: {}
   pullRequestComment:
      matches: "/mything"
   # now the usual presubmit/postsubmit job spec...
   name: cheese
   source: "cheese.yaml"

then folks can trigger a pipeline using any of the supported go-scm webhooks with per-webhook-kind filters