support for proxy contracts for from_etherscan

[mo-anon]

Please beware with me, im new to all this.

What I did

• Modified fetch_abi_from_etherscan to use the ABI of the implementation contract if it is a proxy, enabling the reading of proxy contracts.
• Added a from_etherscan_abi function which returns the actual ABI. I feel like this might be useful (will remove if not needed).

How I did It

Used the Etherscan API to check if the contract is a proxy. If yes, the ABI of the implementation contract is used.
NOTE: Im very positive there are more efficient ways to resolve this as i call the api twice, but it's the only solution I can think of at the moment lol.

How to verify It

Added some simple tests. However, testing/using from_etherscan is a pain because the ABI parser raises an error when the ABI apparently has an invalid length, e.g., uint256[] or address[]. As i see, this is getting fixed (#115)

Description for the changelog

• Added proxy contract support for from_etherscan.
• Added from_etherscan_abi, which retrurns the full ABI of a contract.

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[charles-cooper]

i guess we are going to need add an etherscan api key to the CI

[charles-cooper]

ok i fixed all the little issues. merging, thanks!