fix: support EKS member impersonation

[vgocoder]

Summary

• Avoid client-certificate CSR waits for imported token/exec kubeconfigs that can authenticate with the original credential and impersonate members instead.
• Preserve certificate-based member auth where CSR signing is available, while falling back to impersonation when a member binding has no certificate.
• Reuse the same impersonation-aware access setup across cluster access, proxy, and webkubectl config generation.

Why

Some managed Kubernetes services, including EKS, accept and approve kubernetes.io/kube-apiserver-client CSRs but do not populate status.certificate. KubePi currently waits for the certificate and returns a 500 after the CSR timeout when creating a cluster member.

Test Plan

• go test ./internal/api/v1/cluster ./internal/service/v1/clusteraccess ./internal/api/v1/proxy ./internal/api/v1/webkubectl

Manual Verification

• Reproduced on an EKS cluster: CSR became Approved with empty status.certificate.
• Verified patched KubePi creates an EKS cluster member in ~1s and the generated EKS RBAC allows the impersonated user to list namespaces.