Add CrossDomainGovernor contract

[yosriady]

Context

• We have feed contracts on L2 chains such as Arbitrum.
• In the normal setup, the L2 contracts would be owned and managed by an L2 Gnosis Safe.
• However, there are L2-specific risks that we wish to mitigate, such as:
  1. L2 Seq centralization and its ability to censor
  2. Have an upgrade path if the bridge API breaks, preventing cross-chain messages
• This effort also doubles as an exploration of cross-chain governance (governing L2 contracts from L1.)
• The Governor contract in this PR will own contracts on L2 (similar to Timelock, if you're familiar with that) to mitigate any L2-specific risks.

https://www.notion.so/chainlink/Arbitrum-Optimism-Safe-Options-3dbd7594bc504c2a9aa542cd7955b2e7#6e64dbc8bb8e4948a59d0fd80ca1f6c5

This PR

This PR adds a CrossDomainGovernor contract for Arbitrum (and Optimism.)

• The Forwarder contract is used to receive cross-chain messages from L1 and acts as the owner of L2 contracts.

• It implements a new forwardDelegate function is used to forward Multisend transactions on L2, which can only be triggered by a delegatecall from the Forwarder contract. We use Multisend transactions for our batch Safe proposals.

Here's a diagram how the Forwarder / Governor contract is used alongside Multisend on L2:

Learn more about the above flow in the Spec

[github-actions]

I see that you haven't updated any CHANGELOG files. Would it make sense to do so?

[yosriady]

@krebernisak I've updated the original PR to address your feedback. It now leaves the ArbitrumCrossDomainForwarder used by the L2 Emergency Protocol as-is.

Instead, the new ArbitrumCrossDomainDelegateForwarder contract inherits from the CrossDomainDelegateForwarder base contract and implements DelegateForwarderInterface.

[yosriady]

@krebernisak @se3000 All of the audit feedback has been addressed so I believe we can merge this. Please have another look, thank you.

Resolved