[flash-swap] Add token withdraw method

[scorpion9979]

One implication of #60 being merged is that the flash-swap contract is going to have to contain collateral token balances to insure that it would always have enough collateral to repay the 0.3% Uniswap V2 fee, especially in the case where USDC is used as collateral. Thus, we will need to add a token withdraw method in order for the contract owner to be able to withdraw tokens contained by the flash-swap contract.

[scorpion9979]

This is no longer needed since the bot wallet will contain any USDC needed to pay for the 0.3% Uniswap V2 fee.