fanout (publish only)

[turuslan]

gossip.publish(topic_hash, data) without gossip.subscribe(topic_hash) for publish only mode

[Copilot]

Pull request overview

This PR implements a "publish-only" mode for gossipsub topics, allowing nodes to publish messages without subscribing to a topic. This is useful for fanout publishing scenarios where a node only needs to send messages but not receive them.

Changes:

• Added publish_only_ flag to Topic class to distinguish between subscribed and publish-only topics
• Implemented Gossip::publish(TopicHash, BytesIn) method that creates publish-only topics
• Added fanout TTL-based cleanup for publish-only topics in the heartbeat function
• Skip sending GRAFT/subscription messages for publish-only topics to peers

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
include/libp2p/protocol/gossip/gossip.hpp	Added publish_only_ flag to Topic, last_publish_ timestamp, new publish method, and made meshOutCount() const
include/libp2p/protocol/gossip/config.hpp	Added fanout_ttl configuration parameter for publish-only topic cleanup
src/protocol/gossip/gossip.cpp	Implemented publish-only logic: topic creation, peer selection, mesh management, and cleanup
include/qtils/retain_if.hpp	Added utility function to filter containers in-place based on predicate

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