Feature/l2 balances and transfers 2

[jijordre]

Purpose

Add missing contractAddress property of the Transfer type and prefix types related to ERC20 transfers with "ERC20".

Issues

Closes #32

Check list

• [x] Code runs without regressions even though new code is incomplete
• [x] Automatic tests have been written, they work, and code coverage has been maintained.
• [ ] Code has been submitted to build server and build succeeds
• [x] Code reviewer has been explicitly notified outside automatic notification channels