Get exchange rate CELO/ETH

[karlb]

To calculate the L2 gas cost (which is denominated in CELO), we need to include the L1 gas cost for running the rollup (denominated in ETH). Where could we get the exchange rate to convert the L1 gas cost into CELO? Having the exchange rate is also necessary for #60.

Option 1: Oracle

An oracle could deliver the exchange rate from external sources. This could be data from the L1 that is included in the L1 block data (large delay since we only read confirmed blocks), a service like ChainLink or any other oracle solution.

Option 2: Uniswap pool

Use bridged ETH and native CELO to set up a Uniswap pool on L2 and use that pool as a source. Needs sufficient liquidity for reliability.

[karlb]

According to our current plans, we won't pay any fees in ETH, so this is not necessary.