Add critical flows for Payment Overview currency-switching and total/available balance

[Jinksi]

Merchants need to view their total and available balance for one or more deposit currencies. We currently do not have this defined as a critical flow in the https://github.com/Automattic/woocommerce-payments/wiki/Critical-flows, which are tested during release cycles.

Therefore, we should:

2. Define these flows as critical in the https://github.com/Automattic/woocommerce-payments/wiki/Critical-flows and https://github.com/Automattic/woocommerce-payments/wiki/Testing-instructions-for-critical-flows, and
3. Ensure they are covered with the appropriate e2e and/or integration tests
   • Single currency account balances: #9023 and View account balances critical flow added ✅
   • Multiple deposit currency account balances and switching to be handled in issue #9188

[Jinksi]

Unassigning myself due to focus on multiprocessor work. PR is in a working state and is almost ready to ship.