Structured trigger format

[paoloambrosio-skyuk]

Overview

The design for #217 comes with two issues:

• Validation can get complicated as the number of triggers increases
• The CRD cannot express what fields are available for each trigger type

Design

To solve these problems, we should use a technique similar to what Argo Event TriggerTemplate does.

apiVersion: pipelines.kubeflow.org/v1alpha5
kind: RunConfiguration
metadata:
  name: ...
spec:
  run:
    ...
  triggers:
  - schedule:
      cronExpression: "1 2 3 4 5"
  - change: {}

Validation should be changed to allow only one trigger "type" (field).