Add partial message support for gossipsub

[raulk]

Summary

This PR adds support for partial messages in gossipsub, enabling bandwidth-efficient propagation for protocols like PeerDAS that need to distribute large data in fragments.

Key changes:

• Protobuf extension: Add PartialMessagesExtension message and requestsPartial/supportsSendingPartial fields in SubOpts
• Core interfaces: PartialMessage interface for applications to implement, PartialMessageExtension for router integration
• State tracking: PartialMessageState for tracking message reconstruction across partial RPCs
• Subscription signaling: New subscribePartial() API to indicate partial message support to peers
• Router integration: GossipRouter processes partial subscription options and forwards partial RPCs

Usage

// Create partial message extension with callback
val partialExt = PartialMessageExtension { peerId, partialRpc ->
    // Handle incoming partial RPC
}

// Build router with extension
val router = GossipRouterBuilder()
    .partialMessageExtension(partialExt)
    .build()

// Subscribe with partial support
gossip.subscribePartial(topic)

// Publish partial message
gossip.publishPartial(partialMsg, topic)

Test plan

• Unit tests for PartialMessageState and PartialMessage
• Interop testing with Shadow network simulator
• Integration tests with go-libp2p-pubsub partial messages