This change allows cluster operators to toggle between HTTP and HTTPS
for etcd client connections on a per-cluster basis, matching the pattern
used by Kubernetes GCE scale tests where events etcd uses HTTP while main
etcd uses HTTPS.

Changes:
- Add ClientTLSEnabled field to EtcdClusterSpec (defaults to true)
- Add helper methods IsClientTLSEnabled() and GetClientScheme()
- Update etcd-manager model to use configurable scheme
- Update API server to use configurable scheme for etcd URLs
- Add validation to prevent disabling TLS on main etcd cluster
- Generate deepcopy and conversion functions

Benefits:
- Eliminates TLS handshake overhead for events etcd
- Fixes gRPC connection proliferation issues with TLS
- Allows HTTP for events (ephemeral data) while keeping main secure
- Enforces TLS for main etcd (contains cluster state)

Usage:
  spec:
    etcdClusters:
    - name: main
      # Uses HTTPS by default (secure)
    - name: events
      clientTLSEnabled: false  # Use HTTP (fast)

Signed-off-by: Davanum Srinivas <davanum@gmail.com>