Please open the internal interface Microsoft.Extensions.ServiceDiscovery.LoadBalance.IServiceEndPointSelector

## Background and Motivation

`RoundRobinServiceEndpointSelector` is too simple to select a true endpoint.
For example:
A service instance usually listens multipart ports for many schemes, such as http, https, and so on.
Not only http but also grpc request was sent to service B by service A.
My custom `IServiceEndpointProvider` provides many service endpoints with multipart schemes by the same service name, but the `RoundRobinServiceEndpointSelector` can not select a true endpoint according to the URI scheme.
It is necessary to make `IServiceEndpointSelector` interface public

## Proposed API

```diff
namespace Microsoft.Extensions.ServiceDiscovery.LoadBalance;

public interface IServiceEndpointProvider
{
......
}
```