[selekkala]

Summary

When enable_next_hop_tracking is enabled in the DC/OSS daemon, MainOSS previously passed a null NexthopCache to RibDC. The RIB later dereferenced that cache while processing a route update and aborted.

This change:

• creates the cache only when next-hop tracking is enabled;
• wires connected-neighbor and FIB host-route tracking through NeighborWatcher;
• registers configured static BGP peer addresses before the shared FSDB subscription starts; and
• passes the cache and watcher into RibDC so RIB-IN nexthops are subscribed for tracking.

The existing startup path remains unchanged when next-hop tracking is disabled.

Test plan

• git diff --check
• Attempted the documented DC build with getdeps.py; the host requires sudo for dependency installation, and the non-root retry is missing the gcc12 system-package metadata.
• Existing NeighborWatcherFibTest coverage exercises creation and shutdown of the FIB watcher with next-hop tracking enabled.