Rework the gas limit on Optimism Messengers

[fredo]

We have a constant value of 1M gas as a minGasLimit set in a single variable which both messengers, OptimismL1Messenger and OptimismL2Messenger derive from.

This is fine as long as you don't send a message back to Optimism. This happens in our local e2e tests. This way it is impossible to set the correct value as the message is a nested message send back to Optimism.

In general it would be nice to separate those two. In addition, we could analyze the correct values we need to send messages across.