feat: implement Waku RLN shard as decentralized private mempool

[mfw78]

What problem does this solve?

Centralized private mempools (Flashbots, etc.) create single points of failure and censorship risk. A decentralized private mempool using Waku RLN (Rate Limit Nullifier) would provide censorship-resistant transaction privacy.

Describe the solution

Implement a Waku-based private mempool using RLN shards for spam prevention. Users can broadcast transactions to a decentralized gossip network instead of centralized private mempool providers, maintaining transaction privacy while avoiding centralization.

Alternatives considered

• Only use centralized private mempools (censorship and centralization risk)
• Build custom P2P network (reinventing the wheel, Waku already solves this)
• Use libp2p gossipsub directly (lacks spam prevention mechanism like RLN)

Implementation notes

• Integrate Waku rust bindings for RLN shard participation
• Implement transaction broadcasting to Waku network
• Add RLN proof generation for spam prevention
• Configure appropriate shard for Ethereum transaction gossiping
• Add fallback to centralized private mempools if Waku unavailable
• Consider bandwidth and latency implications vs. centralized alternatives