Improvements from DTL testing

[0xJem]

• Adds canonical Uniswap V2 and V2 addresses to env.json
• Cross-ports salt and test script improvements from the Baseline branch (#155)
• Adds scripts to create FPB auctions with Uniswap V2 or V3 callbacks

[0xJem]

Merging as these are just script updates (no contract code), and to avoid merge hell with the Baseline DTL.