feat: Allow adding dependencies from Etherscan

[antazoey]

What I did

You can now use dependencies from Etherscan
I needed this because I couldn't find some contracts anywhere else

fixes: #57

How I did it

How to verify it

dependencies:
  - name: Spork
    etherscan: "0xb624FdE1a972B1C89eC1dAD691442d5E8E891469"
    ecosystem: ethereum
    network: mainnet

from ape import project

spork_contract_type = project.dependencies["Spork"]["ethereum_mainnet"].Spork

Checklist

• Passes all linting checks (pre-commit and CI jobs)
• New test cases have been added and are passing
• Documentation has been updated
• PR title follows Conventional Commit standard (will be automatically included in the changelog)