[Feature] Make Etcd CRD's spec.backup.store section immutable

[shreyas-s-rao]

Feature (What you would like to be added):
Make Etcd CRD's spec.backup.store section immutable.

Motivation (Why is this needed?):
Make Etcd CRD's spec.backup.store section immutable so that the storage container location isn't allowed to change mid-usage of an etcd, due to potential mismatch of snapshotting and restoration locations, allowing restorations to happen from a different etcd's backup and rendering the shoot cluster unusable. Refer gardener/gardener#4454 for a fix already made on Gardener, although we still want druid to be resilient to potential undesirable changes to the Etcd resource.

Approach/Hint to the implement solution (optional):
Since CRD immutability is yet to be support (refer kubernetes/kubernetes#65973), it might make more sense to use something like a validating webhook on the Etcd resource updates.

/cc @amshuman-kr

[vanjiii]

/assign