Allow signing in to additional account(s)

[cbiesinger]

(somewhat related to #442 )

Some IDPs support being signed in to multiple accounts at the same time. Such IDPs may want to allow the user to sign in to an additional account from the account chooser, i.e. some kind of "Add Account" button (for consistency with the current flow they have)

We should probably support that, perhaps by exposing a new field in the config file

[cbiesinger]

Proposal: add a boolean field allow_use_another_account to the config file. If set to true, the account chooser gets an additional button "Use another account" which triggers the login flow (i.e. opens the login URL in a dialog similar to what the mismatch dialog or the button flow do)

[timcappalli]

2023-11-28 call: exists in Chrome Canary, please test.