Added option to skip gas estimation in order to support transaction batching

[0xdex18]

This PR implements a possibility of skipping the gas estimation for the func returned by generateTxCallback.

This is required for transaction batching, and also useful to skip overhead, in case of smart wallets that process their own gas estimation

this PR goes in pair with our PR of aave/interface , that implements transaction batching (Approval + Transaction) at the same time

Please advise if there's a better way