Add support for /withdrawals/crypto endpoint

[dugjason]

Added

• Support for POST /withdrawals/payment-method endpoint
• Support for POST /withdrawals/coinbase-account endpoint
• Support for POST /withdrawals/crypto endpoint
• Support for GET /payment-methods endpoint
• Support for GET /coinbase-accounts endpoint

[JustinSeidl]

@dugjason I've forked, updated, and created a new Coinbase Pro gem with your withdrawal methods included:

https://github.com/vertbase/coinbase-pro

[drewrothstein]

Hi, we are closing out PRs + Issues as this project is being archived.