Implement retry mechanism for fetching of contract creation tx

[manuelwedler]

From time to time, we have failing tests for fetching the contract creation transaction from etherscan. The Etherscan API is just some times inaccessible. Here is an example for a failing CI: https://app.circleci.com/pipelines/github/ethereum/sourcify/6426/workflows/1bee0997-2d25-4bd9-98fa-13f64ca43d35/jobs/38746

I propose to implement a retry mechanism for fetching the creation transaction. In case an API is simply unavailable for a short time, retrials can make the verification more stable. Retrials with exponential backoff are usually a good idea to not run into rate limiting again and again (here is an example for this technique).