[Identity] Support cross-tenant authentication

[joshfree]

Support the scenario described in https://docs.microsoft.com/bs-latn-ba/azure/azure-resource-manager/authenticate-multi-tenant.

To test it, the shared image gallery scenario takes advantage of this type of authentication header: https://docs.microsoft.com/en-us/azure/virtual-machines/linux/share-images-across-tenants

A previous issue which users were able to workaround: Azure/azure-sdk-for-java#3819. We need to provide a more elegant way.

Related Azure/azure-sdk-for-java#6040

[chlowell]

This feature will be implemented by management clients, not within azure-identity.

[chlowell]

Reopened for tracking.