Rates strategy factory for Aave v2

[Ernesto Boado]

Even if no new listings will happen in instances of v2, it is useful to have an equivalent contract to this one https://github.com/bgd-labs/aave-helpers/blob/feat/draft-engine-0.2/src/v3-config-engine/V3RateStrategyFactory.sol, but for Aave v2.

Things to take into account:

• The default interest rate strategy on v2 is slightly different that v3's https://github.com/aave/protocol-v2/blob/master/contracts/protocol/lendingpool/DefaultReserveInterestRateStrategy.sol
• When initializing() the factory with "live" rate strategies, verify well that none has custom logic (it is possible there are some). If there is any with custom logic, don't list it in the factory.
• No need for config engine support to start with, but include an example payload (probably a base payload exposing a simple interface) to use the factory