[Do not merge] Simulated upgrade test for mainnet Gateway contract

[vgeddes]

Uses Foundry's ability to run tests using storage forked from production networks.

Requirements:

• Add MAINNET_RPC_URL to environment (I've updated Github Actions to include this secret)
• Most recent foundry/forge version

Tasks

• [x] Upgrade production gateway use forking
• [ ] Add sanity tests to ensure upgraded gateway is not compromised (storage corruption, etc).