Support for rollup mode when using fee token

[gvladika]

This feature adds support to run fee token based Orbit chains in rollup mode. Previously only Anytrust mode was supported when fee token is used. This is because ArbO assumes that the asset used to pay for batch posting on parent chain is the same asset which is used as native asset on the child chain. If batch poster spends 0.01 ETH to post batch on the parent chain, it will be reimbursed 0.01 ETH on the child chain. However this approach is not valid when fee token is used. Reimbursing batch poster with 0.01 fee token after it has spent 0.01 ETH would be fine only if exchange rate between ETH and fee token was 1:1. In practice this will not be the case, so we’re introducing a way to get the actual exchange rate on-chain and use it to correctly reimburse the batch poster.

This PR enables SequencerInbox to fetch the feeToken:nativeToken exchange rate from the external source called feeTokenPricer which is set by the chain owner. Pricer needs to implement a single function getExchangeRate() returns(uint256). In practice, pricer can use Chainlink, amm twap, constant price, or any other approach to report exchange rate.