Watchdog should verify published header hash match expected header hash

[mfornet]

If 2/3 of near validators (weighted by stake) collude, they can sign an invalid block that withdraw all the funds from the bridge, without actually broadcasting this block to NEAR network, and pushing this block to NEAR Light Client on Ethereum. If this is done, the watchdog will not be able to detect the malicious behaviour, only from the published data on Ethereum, given it will appear as legit.

Moreover this block can't be challenged.

The watchdog should be connected to a trusted NEAR node, and as part of all the checks, verify the header hash matches NEAR header hash published on the network at that height. If the hash doesn't match, and alert should be triggered, to freeze the bridge. This is becoming more relevant, as the TVL of the bridge increase, an incentives for collusion increase with it.