[BETA API] Implement conversion webhook for alpha-beta transformation

[SaschaSchwarze0]

Provide a conversion webhook for the alpha-beta transformation. https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definition-versioning/#webhook-conversion

This should be a separate binary in this project that produces a separate container image with a separate deployment.

[qu1queee]

@otaviof I send you a msg in Slack, but putting it here for awareness. At the moment I see that this issue could be divided into two PRs:

1. We need to implement a couple of functions(Hub and Spoke), this ones will basically take each of our Custom Resources(Build, BuildRun, BuildStrategy and ClusterBuildStrategy) and convert them to the desired API version(e.g. v1beta1 to v1alpha1). Where Hub is v1alpha1, and spokes is v1beta1. I can cover this via Conversion webhook #1302 .

2. We need a webhook for the conversion. From the kubebuilder docs they place the webhook logic under each API version type, and call it at the init time. In this issue, the proposal is to have this as a single binary with a single deployment. As we could later extend it, to cover other validations/mutations. From the kubebuilder docs, it is stated: This setup doubles as setup for our conversion webhooks: as long as our types implement the [Hub](https://pkg.go.dev/sigs.k8s.io/controller-runtime/pkg/conversion?tab=doc#Hub) and [Convertible](https://pkg.go.dev/sigs.k8s.io/controller-runtime/pkg/conversion?tab=doc#Convertible) interfaces, a conversion webhook will be registered., this means to me that we do not need to specify anything on the webhook regarding conversion(I might be wrong), but we just need a webhook that can mutate our custom resources(I think 4 different endpoints).

@otaviof you can move on with 2, and then I do 1. We can exchange notes over the next days.

[qu1queee]

We had a session on this issue today, here a list of items:

• The conversion webhook implementation is now being done without the https://pkg.go.dev/sigs.k8s.io/controller-runtime/pkg/conversion#Convertible interface. The reason is that the kubebuilder tutorial webhook setup did not work for us. Hard to say why, but the webhook was never serving the conversion. So we decided to adopt the more native approach which implies a little bit more of code.
• Due to the above, @qu1queee to reflect that in the original SHIP.
• Unit test are missing at the moment for the conversion.
• We discussed on how to autogenerate the CABundle, for both the ValidatingWebhookConfiguration and the CRDS(where we need the spec.conversion block. @SaschaSchwarze0 to open a new issue on this.
• Currently , the webhook is not doing the conversion properly, so it remains as wip.