[Feature] Enhance the Etcd resource status structure as proposed in the multi-node proposal

[amshuman-kr]

Feature (What you would like to be added):

Enhance the Etcd resource status structure according to the changes proposed here while maintaining backward compatibility for the consumers of Etcd resource status (such as the gardenlet).

Motivation (Why is this needed?):

Pick individually executable pieces of the multi-node proposal.

Approach/Hint to the implement solution (optional):

For backward compatibility, the existing status fields and the values in them need to maintained as they are. In both the main etcd-druid controller (especially, here and here) as well as the newly separated custodian controller.