feat: proposer forwarder contract

[just-mitch]

Due to a dos protection in g/reth, we cannot submit normal and blob transactions from the same L1 account in the same L1 block.

This means we cannot submit governance votes and propose L2 blocks in the same L1 block.

The solution is to provide a thin "Forwarder" contract which operators can specify as their proposer address: effectively an account abstraction providing multicall.

Then, the sequencer can "bundle" the appropriate actions it wants to take during each L1 slot into a single transaction, which are dispatched by this Forwarder.

[spalladino]

This will also come in handy with cancellations. We don't have to actively cancel a blob tx to free up a nonce, we can just move to another EOA and let the old one be dropped.