feat(CCHAIN-1105): Add GossipSub explicit peering

[ancazamfir]

Closes: #XXX

Summary

Add support for GossipSub explicit peering to ensure reliable message delivery between validators, bypassing mesh formation dynamics.

• Add enable_explicit_peering and enable_flood_publish configuration options to GossipSubConfig
• When explicit peering is enabled, persistent peers are automatically added as explicit peers in GossipSub
• Add explicit_peers metric to track active explicit peer connections
• Force flood_publish=false when explicit peering is enabled (with warning if overriding config)

GossipSub mesh formation is probabilistic and can result in validators not being directly meshed with each other, requiring messages to traverse through full nodes or rely on IHAVE/IWANT gossip.

Configuration

Enable via environment variable:

MALACHITE__CONSENSUS__P2P__PROTOCOL__ENABLE_EXPLICIT_PEERING=true

Or in config.toml:

[consensus.p2p.protocol]
enable_explicit_peering = true

Testing

• Start testnet with explicit peering enabled
• Verify validators have zero mesh peers but consensus progresses
• Check that explicit_peers metric shows expected connections

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