Implement the BalancerExchnageProvider.sol

[philbow61]

Description The BalancerExchangeProvider will be a contract implementing the IExchangeProvider interface. This Exchange will use the Bancor logic to price swaps. The BalancerExchangeProvider will use the Power.sol contract for the exponential calculations.

For a more detailed description of the necessary functions and math check out the notion page of this Epic.

Acceptance Criteria

• [ ] IBalancerExchangeProvider.sol interface is created
• [ ] BalancerExchangeProvider.sol is implemented
• [ ] BalancerExchangeProvider is tested