Implement admission webhook

[Gerrit91]

This would be nice in order to move the existing validations into a dedicated place where it already prevents the creation / update of the respective Kubernetes resource.

Inspiration can be gathered from:

• https://github.com/metal-stack/gardener-extension-provider-metal/blob/master/pkg/admission/validator/webhook.go
• https://github.com/gardener/gardener-extension-registry-cache/blob/main/pkg/admission/validator/cache/shoot.go