[Core] Allow disabling subscription finding

[jiasli]

This is previously raised by IcM 154830377: Azure CLI fails if NSG doesn't allow ARM endpoint

It should be possible to force az login to only do a tenant-level login and disable subscription finding. This is also called ARM-detached mode.

The current param --allow-no-subscriptions always queries ARM and the result depends on whether the tenant has subscriptions. The result is unpredictable for the client:

Does the tenant have subscriptions?

• ✔ -> build subscription-level accounts
• ❌ -> build a tenant-level account

Allowing disabling subscription finding is helpful in these scenarios:

1. As in the IcM, if ARM endpoint (https://management.azure.com/ in public AzureCloud) is blocked by an NSG, az login fails. However, it shouldn't, as customer should still be able to use az to manage data-plane services like Storage, Microsoft Graph...
2. A custom cloud may not have ARM endpoint, thus causing az login to fail

[jiasli]

Introduced by #13053, Profile.login now has a flag find_subscriptions to disable subscription finding.