Implement helpers for withdrawing ETH and ERC-20 tokens

[PaulRBerg]

These should go in a dedicated targets package after I switch this repo to a monorepo structure:

Refs:

• https://github.com/cantinasec/review-sablier2/issues/21
• https://github.com/PaulRBerg/prb-proxy/issues/112

[PaulRBerg]

WIP on the feat/withdawer-target branch.