Emit event when validator contract address is set in bridge contract initialization

[patitonar]

There could be a rare case where the validator contract address needs to be changed in the bridge contract. For example:

Imagine that the bridge has lost the control over the validators contract e.g. the validators contract upgradability admin is not the same account as the bridge upgradability admin AND the private key of the validators contract upgradability admin was lost.
So, we will need to stop the bridge and deploy another validator contract with the same parameters as an old one except for the validators contract upgradability admin account.

It makes sense to add a new event like ValidatorContractChanged to be able to get the historic values of the validator contracts addresses in case it changes.