Support ARM clients with CloudConfiguration::Custom

#3087 added an API that handles data plane clients and suggests a way to extend `CloudConfiguration::Custom` to handle the ARM case as well: add a field like `resource_manager_endpoint: Option`. This seems reasonable because one ARM endpoint suffices for an entire cloud; it's a top-level concern like `authority_host`.

Start by reading the API introduced in #3087 and locating CloudConfiguration::Custom and its data-plane client usage. Trace how authority_host is handled, then determine where a resource_manager_endpoint field would be carried and consumed; done means ARM clients can use the configured endpoint while existing Custom configurations remain supported.

Written by the indexing model from the issue text.