auth: Support multi-account scenarios

[alexweininger]

TODO: bump version appropriately

The changes are basically adding an account parameter wherever there's a tenant parameter. The rest of the changes are fixes for issues that came up with the new multi-account scenarios. The changes in this PR have been tested by two partner teams, Bicep and SCOPE studio.

Fixes:

• Added a new call to getSubscriptionsForTenant that gets subscriptions in the users home tenant. This can mean duplicates show up in the initial result so I added logic to remove them.
• Added logic to remove duplicate subscriptions while retaining those from different accounts.
• Enhanced the isSignedIn method to check for a session across all accounts if no tenant or account is provided.
• Refactored the getSubscriptionsForTenant method to ensure it returns an empty list if the user is not signed in to the specified tenant or account.

[alexweininger]

These are big changes with potential to change behavior, but there are no breaking API typings changes. Should I bump to 4.0.0 or to 3.1.0?

[bwateratmsft]

These are big changes with potential to change behavior, but there are no breaking API typings changes. Should I bump to 4.0.0 or to 3.1.0?

That's a good question. I'd lean toward 3.1.0 since there aren't strictly breaking changes.