Validate upgrade requests

[adrianmo]

At this moment, upgrade requests are not being validated, meaning that the operator will always trigger an upgrade process to any version specified in the resource definition.

We need to check if the version is explicitly supported. The operator should not attempt to upgrade to a non-supported or non-compatible version. Upgrade support to a particular version needs to be implemented and added to the operator. Furthermore, some versions might require specific actions and verifications, like applying migrations.

This could be implemented as an admission webhook as part of #7.

Related documentation:

• https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/#admission-webhooks
• https://github.com/kubernetes-sigs/controller-runtime/blob/master/example/validatingwebhook.go
  Admission Controller for PravegaCluster resource #7

[adrianmo]

Related to #7