Topology with remote datacenters too

**Old UI or New UI**

New UI with Consul 1.9.0-beta3.

**Describe the problem you're having**
The topology UI is a really interesting view to help people understanding how the mesh connections are linked. There is a little _information_ icon on the downstream which let you know what only services within the same datacenters are shown.
On the upstream (right side), there's isn't such icon, so I was hopping to see all upstreams, even the one declared within another datacenter.

In my example I have a _ratings_ service declared in another DC _native2_ which isn't shown:

I can see the API endpoint does not return the list of upstreams from another DC.

**Describe the solution you'd like**
I would like to see the upstreams services in any datacenter, not only the local one. Simply have extra boxes on the right hand side per datacenter with their respective services in it.

Start at the topology UI and the API endpoint that currently returns upstreams, then trace where results are limited to the local datacenter. Done means upstream services from other datacenters appear in separate right-hand boxes with their respective services, while preserving the existing topology view.

Written by the indexing model from the issue text.