Added dydx interface and interface folder for third-party devs to easily find and work with dydx

[mikedeshazer]

Added dydx interface and interface folder for third-party devs to easily find and work with dydx

[Akhilleshgoswami]

@mikedeshazer I want to ask you something can you let me know can we access the functions of the dydx on our own smart contract because when I asked at dydx discord they said this is not possible right now.