SyncNode as the AMB bridge validator

Consider to add SyncNode's account 0x19aC7c69e5F1AC95b8d49b30Cbb79e81f1ab0dba to the AMB bridge validators set. This action is aimed to increase security of the bridge operations.

SyncNode is already the validator of the xDai bridge, the xDai network validator and also participates in the xDai Bridge governance Gnosis Safe.

After adding SyncNode's account to the AMB bridge validators set, it is also proposed to increase required number of confirmations for transferring messages through the bridge as so 4 of 6 validators will be required to act to execute transfers between the Ethereum Mainnet and the xDai TokenBridge.

This proposal is created mainly to expose the decision that needs to be made. It means that even there is no vote for this proposal here, the agreements could be still provided through the confirmations in the Gnosis Safes.