[Feature] Add validation for etcd resources

[gardener]

[Feature] Add validation for etcd resources

[stoyanr]

/cc @amshuman-kr @plkokanov @vanjiii

[amshuman-kr]

Thanks for creating the issue @stoyanr! @shreyas-s-rao created another issue #213 for the same purpose. But the approach is slightly different that it is suggesting to make the backup configuration fields immutable which will have much of the same effect as the validation proposed here. Should we keep and address both issues? WDYT?

[stoyanr]

Sorry @amshuman-kr I didn't see that issue yesterday. I think both approaches are valid and should be considered. Validation of etcd resources is needed for both of them, so the 2 issues are somewhat duplicates. Feel free to close one of them if you prefer.

[vanjiii]

/assign