Fix base path for appstore

[SerKnight]

• Fix base path for app store

Implements the endpoints available in Xero's API spec.

Description

https://developer.xero.com/documentation/guides/how-to-guides/xero-app-store-subscriptions/

https://developer.xero.com/documentation/api/xero-app-store/subscriptions

Release Notes

Subscriptions endpoints enable you to query data about the users who have paid to subscribe to your application through Xero.

AppStoreAPI adds

• getSubscription

• AppStore:: adds

  • Subscription
  • SubscriptionItem
  • Plan
  • Product
  • Price
  • ProblemDetails

• PayrollAU Employee adds termination_reason V Voluntary cessation I Ill health D Deceased R Redundancy F Dismissal C Contract cessation T Transfer

• PayrollAu::PayrollCalendar adds reference_date

• Payment adds bank_amount

Types of Changes

• [x] New feature (non-breaking change that adds functionality)