Define a clear user interface for accessing the etcd clusters provisioned by etcd-operator

etcd-operator provisions and manages etcd clusters. However, the ultimate goal is not only to create and operate etcd clusters, but also to ensure users can reliably and securely access them. Therefore, we need to define a clear user-facing interface for accessing etcd clusters provisioned by etcd-operator.

Two key aspects need to be addressed:

- We should provide a stable Service/endpoint for users to access each etcd cluster. The endpoint should remain stable regardless of the underlying Pod lifecycle.
- We should provide client certificates for each etcd cluster so users can connect directly. Alternatively, we should expose an interface that allows users to request or generate client certificates when needed.

Start by reviewing the operator’s existing cluster and Service provisioning behavior, then trace how users currently obtain endpoints and credentials. Define the stable endpoint and client-certificate interface, with acceptance criteria covering Pod replacement, reliable access, and secure client connections.

Written by the indexing model from the issue text.