Update QUIC flood control for shared validator IPs

[EmelyanenkoK]

This change fixes the case where multiple validators behind the same IP interfere with each other due to QUIC flood control.

It makes two changes:

1. RFC1918 private IPv4 ranges are no longer subject to per-IP QUIC flood control:

• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16

2. Validator endpoints can use separate flood buckets by exact ip:port, so multiple validators on the same public IP no longer
   consume the same per-IP flood quota.

The global new-connection limiter is unchanged.

What changed

• QuicServer flood control is now bucket-based:
• RFC1918 source -> bypass per-IP flood bucket
• protected validator endpoint -> bucket by exact ip:port
• all other traffic -> bucket by ip
• The selected flood bucket is stored on accepted inbound connections and reused on close, so accounting stays correct even if
  protection state changes later.
• Added protected-endpoint registration in QUIC.
• QuicSender now tracks protected validator peers per local ADNL ID, resolves their exact endpoints, and updates the QUIC server
  with those endpoints.
• ValidatorGroup now registers current validator-set peers as protected on session start and unregisters them on destroy.
• Added CLI/config knobs in validator-engine:
• --quic-exempt-private-rfc1918-from-per-ip-flood
• --quic-protect-validator-endpoints-from-shared-ip-flood

Both are enabled by default.

Notes

• This change does not increase the numeric flood limits.
• It changes how inbound QUIC connections are bucketed for flood-control purposes.
• The scope of protected peers in this patch is the current validator set.

[github-actions]

1. Medium: protected peer endpoint resolution has no retry path, so a transient get_peer_node/address miss leaves that peer permanently unprotected unless an outbound reconnect happens later.
   quic/quic-sender.cpp:485 returns immediately on error, and there is no periodic/backoff re-resolution for entries still present in protected_peers_. This can silently defeat the new shared-IP protection for inbound-only peers.

2. Medium: RFC1918 exemption bypasses both concurrent flood buckets and per-bucket new-connection rate limiting, not just per-IP bucket sharing.
   quic/quic-server.cpp:232 returns nullopt for private IPv4, and quic/quic-server.cpp:247 then skips conn_rate_limiters_.take_new_connection(...). On private but untrusted networks, this creates a much weaker limiter than before (only global limiter remains). Consider keeping a per-endpoint limiter for exempt ranges if that is not intentional.