[Chain Orchestrator] Improve `UpdateFcsHead` method to add L2 transactions to transaction pool

[frisitano]

Overview

Currently, we implement a relatively naive solution for the UpdateFcsHead rpc method. This involves two steps:

1. We purge all mappings of L1 messages -> l2 blocks (effectively marking them suitable for inclusion in future blocks.
2. Updating the head in the fork choice state.

@Thegaram proposed a more elegant solution in which we extract all L2 messages from the reverted chain and push them into the transaction pool such that they can be included in future blocks.