[Enhancement] Decouple the etcd member name from the pod name

[acumino]

How to categorize this issue?

/area control-plane-migration high-availability ops-productivity performance robustness scalability usability
/kind enhancement

What would you like to be added:
Currently, the etcd member name is the same as the pod name of the member.
Because of this, in scenarios like Live CPM, where we want to have multiple etcd members with the same pod name, the new member cannot join the cluster.

Therefore, we want the etcd member name to be generated independently of the pod name.

Why is this needed:
The supports use cases like Live CPM

[gardener-robot]

@acumino No more than 5 labels permitted, but 7 labels were given.

[acumino]

/area control-plane-migration ops-productivity performance robustness scalability

[acumino]

cc @unmarshall @shreyas-s-rao @seshachalam-yv

[ishan16696]

/assign @CaptainIRS