Maximum number of validators handled by a fee manager

[akolotov]

As per the security audit there is a recommendation to explicitly limit the number of iterations assuming that it is not possible to distribute fees to number of validators that exceed some X:

https://github.com/poanetwork/poa-bridge-contracts/blob/786be68cd69526e561dd5654844ac373cda7a539/contracts/upgradeable_contracts/ValidatorsFeeManager.sol#L41-L53

There is no simple way to say what should be a limit since the method refers on the functionality implemented in a particular fee manager.

Let's think of an approach that could estimate possible gas usage of the method onFeeDistribution and recommend a maximum of the validators that can be used for the particular bridge. For example, it could be a script that will deploy particular set contracts (as per the user deployment configuration) but in a local testnet, then the script will run some tests and estimate maximum number of validators handled by distributeFeeProportionally, then this estimation should be recommended to be set up in BaseBridgeValidators to limit _addValidator method properly.