Create a new account for existing customers in the bank

[huangweili97]

As a existing bank customer, I want to create a new account so that I can deposit and withdraw money from account

Acceptance Criteria

Scenario 1: successful account created
Given the user has a verified username and are of age greater than or equal to 18
When they request a new bank account
Then a new bank account is created with a unique 10-digit id and zero balance

Scenario 2: unsuccessful account creation due to customer being below 18
Given the user has a verified username but is below 18
When they request a new bank account
Then no bank account is create and an error message is displayed

Scenario 3: unsuccessful account creation due to invalid username
Given the user does not have a verified username and is above 18
When they request a new bank account
Then no bank account is create and an error message is displayed

Tasks

1. Create a class for a bank that can maintain accounts and usernames
2. Create a method for creating a new bank account and adding it to the bank's existing accounts
3. Create tests to verify AC