/kind design

What this PR does / why we need it:

Adds the enhancement document for an internal iBGP Route Reflector running on control plane nodes.

In cloud environments where the ToR does not support EVPN, east/west traffic cannot be distributed via the fabric. This enhancement adds an internal RR so EVPN routes are reflected between router pods and VXLAN data plane goes directly node-to-node, avoiding the ToR.

Design highlights:

• RR runs as a DaemonSet on control plane nodes (stable, known IPs, no rescheduling)
• Two modes based on runOnMaster:
  • true: reuse existing router pods + hostcontroller on CP, add RR config via RawFRRConfig
  • false: deploy additional RR-only router pods on CP with a separate Underlay CR (NIC + ASN, no ToR, no EVPN data plane)
• Client discovery: users manually add CP node IPs as iBGP neighbors (same ASN) in the Underlay CR
• Inter-RR peering: automatic — RR controllers discover each other on CP nodes
• bgp listen range on RR side for accepting dynamic clients
• allowas-in only on eBGP neighbors (template conditional fix needed)
• No auto-discovery mechanism, no underlay-ips annotation, no hostcontroller cache extension

Special notes for your reviewer:

This is the enhancement document only, no code changes.

Release note:

NONE