Feat.Req - Address frontrun security

[hopeyen]

Due to the preferred restriction of 1:1 relationship between indexer and Graphcast agent, there exist a vulnerability for indexers who submits a transaction to set their Graphcast agent, in which a malicious actor can frontrun transactions to disabled someone from setting the desired agent address.
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POC

• Alice wants to set her operator to 0x1234.
  She calls setGossipOperator(0x1234).

• ​Malicious Bob sees this in the mempool and sends another setGossipOperator(0x1234) with higher gas price.

• After Bob's tx succeeds, Alice's will fail because operatorRegistered[0x1234] is already set.

Potential solutions

1. commit/reveal scheme like ENS: first transaction with hash of gossip address, second transaction reveals the preimage and claims it
2. Instead of requiring the address field for Graphcast operator, instead require a message signed by the operator private key. Message object should contain both operator address and the message sender address. The contract verify the msg.sender and message signature