Set up defaulting webhook

[danielvegamyhre]

Fixes #6

• Adds defaulting and validation webhook
• Defaults job.spec.completionMode to Indexed if it is unset (includes unit tests and integration tests for this)
• Currently the webhook does no validation; all validation is still done via CEL
• Manually tested by deploying on a cluster and creating a JobSet with a job that has completionMode unset, then verifying the job was created in Indexed completion mode.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: danielvegamyhre

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [danielvegamyhre]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[danielvegamyhre]

I was not able to use wrappers here because it would create a circular import dependency (testing package imports v1alpha1, so v1alpha1 cannot import testing).

We could consider a refactor to solve this (kueue uses a separate webhooks package and sets up a CustomerDefaulter, which allows for more flexibility, contextual logging, etc. but this would require creating a separate webhook struct rather than adding methods to the JobSet struct to implement the Defaulter interface, and it becomes more complicated and didn't seem like it was worth it for this very simple defaulting we are doing.

[ahg-g]

/label tide/merge-method-squash

[ahg-g]

/lgtm