feat: add base mainnet support

[dtbuchholz]

Summary

Adds Base mainnet support. Closes ENG-870.

Details

• Adds new chain for base, chain ID 8453, contract at 0x8268F7Aba0E152B3A853e8CB4Ab9795Ec66c2b6B
• Adds Base for associated deployments helper contract.

How it was tested

Deployed to mainnet at link above.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[dtbuchholz]

@avichalp contract info above for the validator release

[dtbuchholz]

@joewagner needa 👍 again after rebasing (and adding the base mainnet value to the readme, too)