New v3 API endpoints for bank account users

We have introduced a set of API endpoints for new bank account user operations. You can now nominate and archive bank account users.

When you nominate a bank account user, BILL sends the user an email asking them to sign in to the BILL web app and complete identity verification. After the verification is successful, the bank account user can pay with the specified bank account.

When you archive a bank account user, the user loses the ability to pay with the specified bank account.