Feature: support for multiple Google accounts

Describe the feature you'd like to request

I have two GSuite accounts (personal and my employer). Currently, this app only supports one account, which means that I can't sync my work calendars etc to Nextcloud. So, I cannot use Nextcloud Cal to view my agenda, I have to use the Android GCal app to view all my calendars in one place.

This feature has been requested on the Nextcloud forum as well: https://help.nextcloud.com/t/google-data-migration-multiple-accounts/104065

Describe the solution you'd like

Enable users to add multiple API keys under "Connected Accounts > Google Integration".

or

Enable users to authenticate multiple accounts under "Data migration > Authentication"

Describe alternatives you've considered

• I repeated the directions for creating a new OAuth application on my employer's GSuite account, but there is no place to put the second set of credentials under "Connected Accounts > Google Integration".
• I edited the original OAuth application such that my work email was an authorized test user. However, the current setup page under "Data migration > Authentication" does not have an option to authenticate additional accounts
  

Start at the Connected Accounts > Google Integration and Data migration > Authentication entry points described in the issue, and trace how one Google account is currently authenticated and stored. Define the account model and UI flow needed for adding multiple accounts, then verify that users can authenticate and sync calendars from both accounts without replacing the first.