Plaid Retrieve real-time balance data

The `/accounts/balance/get` endpoint returns the real-time balance for each of an Item's accounts. While other endpoints, such as `/accounts/get`, return a balance object, only `/accounts/balance/get` forces the available and current balance fields to be refreshed rather than cached. This endpoint can be used for existing Items that were added via any of Plaid’s other products. This endpoint can be used as long as Link has been initialized with any other product, `balance` itself is not a pro...