CRD validation for EtcdCopyBackupsTask

[unmarshall]

How to categorize this issue?

/area control-plane
/area open-source
/kind enhancement

What would you like to be added:
Introduce CEL validations for EtcdCopyBackupsTask CRD. In #989 CEL validations were introduced for Etcd CRD. There is a need to also do the same for EtcdCopyBackupsTask CRD.

Why is this needed:
To ensure that invalid EtcdCopyBackupsTask CR's are checked even before an event is raised in the respective reconciler in etcd-druid.