[refactor] make BGP peer and secret management system event-driven

[aritrbas]

Summary

This PR refactors the BGP peer and secret management system from a tightly-coupled handler-based architecture to a loosely-coupled event-driven system.

Changes by Component

1. PeerWatcher (watchers/peers_watcher.go)

• Added local peer cache: cachedPeers map[string]calicov3.BGPPeer
• Added granular events:
  • PeerAdded - single new peer added
  • PeerUpdated - single existing peer updated
  • PeerDeleted - single existing peer deleted
  • PeersChanged - for initial state reconciliation only
• Updates SecretWatcher proactively on peer list changes
• Achieves O(1) cache lookups instead of O(N) API list calls

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2. SecretWatcher (watchers/secret_watcher.go)

• Removed GetSecret() method (blocking) with events
• Removed SweepStale() method (replaced with OnPeerListUpdated)
• Emits granular events:
  • SecretAdded - new secret available
  • SecretChanged - existing secret modified
  • SecretDeleted - existing secret removed
• Proactive secret watching via OnPeerListUpdated() - starts watches when peers are added
• Automatic cleanup of stale secret watches when peers are deleted

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3. PeerHandler (routing/peer_handler.go)

• Added local secret cache: secretCache map[string]map[string]string
• Added granular peer event handlers:
  • OnPeerAdded(peer) - O(1) processing for single peer
  • OnPeerUpdated(old, new) - O(1) diff and update
  • OnPeerDeleted(peer) - O(1) removal
• Added granular secret event handlers:
  • OnSecretAdded(secretData) - cache secret locally
  • OnSecretChanged(secretName) - mark affected peers for update
  • OnSecretDeleted(secretName) - remove from cache
• Added ProcessPeers() for initial reconciliation only

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4. FelixServer (felix/felix_server.go)

• Registered for new event types: PeersChanged, PeerAdded, PeerUpdated, PeerDeleted, SecretAdded, SecretChanged, SecretDeleted
• Acts as an event coordinator (no business logic); routes events to appropriate PeerHandler methods

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Made the changes on top of the nsk-split-svc branch for now, will rebase on top of master once those commits for single thread agent are merged.

[sknat]

Many thanks for taking a stab at this !
A couple comments inline, I think there is still room for more untangling of the logic.
Tell me if anything sounds unclear