YARP integration: unable to proxy to an external service running on localhost

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

If a developer wants to use an existing endpoint serving from "localhost" address with YARP. it is unclear how to make it happen. I tried two ways and in both cases YARP tried to find the endpoint on its inner container network, which obviously failed.

### Expected Behavior

_No response_

### Steps To Reproduce

1. Create Aspire starter app
2. Start a "hello world" type of HTTP service listening on localhost address, port 8080 (change port as necessary).
3. Add the following to the app code

```csharp
var hellow = builder.AddExternalService("hellow", "http://localhost:8080");

builder.AddYarp("yarp")
.WithExternalHttpEndpoints()
.WithConfiguration(yc =>
{
var c = yc.AddCluster(hellow); // Also tried using hellow directly, which also compiles and runs
yc.AddRoute("/hellow/{**catchall}", c).WithTransformPathRemovePrefix("/hellow");

});
```

4. Start the app and try to hit the YARP endpoint path `/hellow`. YARP returns 502 Bad Gateway. In the error logs you will find that it tires to connect to "localhost:8080" (on the CONTAINER loopback device, not on host) and does not find anything listening on that address/port combination.

### Exceptions (if any)

_No response_

### .NET Version info

_No response_

### Anything else?

_No response_

Start with the Aspire starter app reproduction using AddExternalService("hellow", "http://localhost:8080") and the AddYarp configuration shown in the issue. Run the service and inspect YARP's 502 error logs to trace how localhost is resolved; done means the /hellow route successfully proxies to the external host service rather than the container loopback.

Written by the indexing model from the issue text.