Implement rate provider contract for Balancer integration

[ghost]

Is your feature request related to a problem? Please describe.

In order to launch mevETH pools on Balancer, we’ll need a rate provider contract that will expose the mevETH price in ETH via a getRate() function. See here: https://docs.balancer.fi/reference/contracts/rate-providers.html

Additional context

As a further reference, here’s the wstETH/ETH rate provider contract: https://etherscan.io/address/0x72D07D7DcA67b8A406aD1Ec34ce969c90bFEE768#code

[sandybradley]

This is quite basic. Let me implement a draft for us.

[sandybradley]

https://github.com/manifoldfinance/mevETH/pull/72