Spike: Create admission webhook for CRD validation

[nilsgstrabo]

Today, validation of RadixRegistration, RadixApplication and RadixDeployment is done by pipeline og radix-api by directly calling function in the radixvalidators package. When we update the validation rules, we must also update the reference in radix-api and rebuild. the validation functions also require various configurations, like DNSAlias reserved list, which means that we must pass a lot of unnessesary information through envvars to the callers.

An admission webhook will remove the need for calling the methods directly and passing configuration to the callers. It is the webhook's responsibility to perform validation. If a caller (pipeline runner or radix-api) tries to create or update a resource with invalid config, the webhook will reject it.

The Kubebuilder project has a CLI that we can use to create webhooks

Timebox: 2 days 1-2 pers