Introduce a validating webhook for TaskDefinitions

[RealAnna]

Details

With #1421 we introduce multiple specs in the TaskDefinition CRD, so that the user can either declare a task using JS or a custom container. Since the TaskDefinition CRD is becoming more complex, we should introduce a validating webhook to alert users immediately if the CR being applyed is wrongly configured.

AC

• use kubebuilder to generate a validation webhook for TaskDefinition (you can have a look at feat: add validating webhook for Keptn Metrics #668 to see what is needed for one)
• the webhook verifies that Exactly one spec should be defined in a TaskDefinition, this could be a custom container spec is filled in OR a JS function spec (OR if already implemented a Python function spec) but NOT both or more. Also, at least one should always be filled in for the TaskDefinition to make sense, otherwise, the job would run nothing.
• integration test to prove the webhook works (at least one test assertions that expects failure and one that passes) you can find an example of error assertions in kuttl here

DoD

• applying a TaskDefinitions with multiple or no runtime spec fails with a clear K8s API message

Resources

• Validation webhook example: feat: add validating webhook for Keptn Metrics #668
• How to test a validation webhook: https://github.com/keptn/lifecycle-toolkit/pull/668/files#diff-e569f1a0298445ca050856250a1a1e179f6a3dea768447b9256df3ec189080e7R3

Depends on

• Integrate the Python runtime runner into the task Controller #1490
• Create Container Runtime for Tasks  #1421

[geoffrey1330]

I'm Interested In working on the issue