Implement custom validation checks

[jkupferer]

Custom validation checks should process using jinja2 to validate check conditions:

spec:
  validation:
    customValidation:
    - name: destroy schedule must be no more than 14 days from creation
      check: spec.actionSchedule.destroy < timestamp(resource_claim.metadata.creationTimestamp).add('14d')
    - name: stop schedule must be no more than 8 hours from now
      check: spec.actionSchedule.stop < timestamp.utcnow.add('8h')