HealthMonitor: Add topology-aware netsplit detection and warning

HealthMonitor: Add topology-aware netsplit detection and warning

Problem:
Currently, Ceph cannot detect and report network partitions (netsplits)
between monitors in different topology locations in a consolidated way.
While stretch mode can handle partitions through monitor elections,
users lack visibility into the topology-level view of network
disconnections, making troubleshooting difficult.

Solution:
This implementation adds a hierarchical netsplit detection mechanism that:
- Uses DirectedGraph structure for netsplit detection
- Maps monitor disconnections to relevant CRUSH topology levels
- Aggregates individual disconnections into location-level reports when appropriate
- Detects complete location-level netsplits when ALL monitors between locations
  cannot communicate
- Reports specific topology locations experiencing complete communication failures
- Falls back to individual monitor-level reporting for partial disconnections
- Handles monitors with missing location data gracefully
- Leverages HealthMonitor::check_for_mon_down to receive a set of down monitors,
  efficiently avoiding false netsplit reports for monitors already known to be down
- Implements smart filtering that correctly excludes down monitors from location-based
  analysis, ensuring accurate netsplit reporting at both individual and topology levels

The implementation produces user-friendly health warnings:
1. For complete location netsplits: "Netsplit detected between dc1 and dc2"
2. For individual monitor disconnections: "Netsplit detected between mon.a and mon.d"

Performance considerations:
- Time complexity: O(m²) where m is the number of monitors
- Space complexity: O(m²) for connection tracking
- Practical impact is minimal as monitor count is typically small (3-7)

Fixes: https://tracker.ceph.com/issues/67371
Signed-off-by: Kamoltat Sirivadhna <ksirivad@redhat.com>
Conflicts:
	src/mon/Elector.cc - Trivial Fix