Add Whitelist contract

Subtask of #6 

For building and running tests of transaction with fee currencies, we need the whitelist contract to query valid fee currencies.

```[tasklist]
### Tasks
- [ ] Add the whitelist contract to the https://github.com/celo-org/optimism repo
- [ ] Adapt the import script and generate wrappers for it: https://github.com/celo-org/op-geth/tree/celo1/contracts/celo
```
