Support referencing existing Service Bus resources in references

[captainsafia]

Opening this PR to sanity check the approach before adding it to other resource types. Some things of note:

• Support for referencing existing resources in other subscription IDs/resource groups isn't implemented due to a gap in the underlying provisioning APIs. See [FEATURE REQ] FromExisting methods should support setting scopes Azure/azure-sdk-for-net#47980. There are workarounds for the but the juice might not be worth the squeeze.
• Separate APIs for publishing/running with an existing resource backed by an annotation and explicit checks in the ConfigureInfrastructure calls for each resource.

[davidfowl]

Opening this PR to sanity check the approach before adding it to other resource types. Some things of note:

This should be a draft right?

[eerhardt]

(nit) Is this test name correct? the test calls RunAsExisting

[captainsafia]

Yeah, the test name is correct although confusing. I was intending to say that you need to call PublishAsExisting in publish-mode otherwise the API effectively no-ops.

[eerhardt]

Does generating these Queues and Topics work with an existing resource?