feat(contract-verifier): Support verification requests for contracts verified on Etherscan

[popzxc]

Prerequisite for matter-labs/block-explorer#359 (see the issue for more details).

We need to support a new form of verificaiton request which would indicate that the contract is verified on Etherscan.
For such a request, contract verifier can fetch source code from Etherscan API (this assumes that URL for the corresponding Etherscan implementation and API key are part of the configuration) and turn it into a verification request.

The verification should not be trustless, e.g. we indeed need to make sure that the results match the deployed bytecode.