refactor load balance client so that users can add their custom implementation

[liubao68]

Is your feature request related to a problem? Please describe.

My application needs to add a custom Client like FeignBlockingLoadBalancerClient, e.g.

public class CustomFeignBlockingLoadBalancerClient implements Client {
  ...
}

CustomFeignBlockingLoadBalancerClient will add some extensions to FeignBlockingLoadBalancerClient, like tracing.

This works pretty well when using load-balanced clients. But when mixed with load-balanced and non-load-balanced clients, FeignClientFactoryBean hard coded to work with FeignBlockingLoadBalancerClient to get the delegate

		if (client != null) {
			if (client instanceof FeignBlockingLoadBalancerClient) {
				// not load balancing because we have a url,
				// but Spring Cloud LoadBalancer is on the classpath, so unwrap
				client = ((FeignBlockingLoadBalancerClient) client).getDelegate();
			}
			if (client instanceof RetryableFeignBlockingLoadBalancerClient) {
				// not load balancing because we have a url,
				// but Spring Cloud LoadBalancer is on the classpath, so unwrap
				client = ((RetryableFeignBlockingLoadBalancerClient) client).getDelegate();
			}
			builder.client(client);
		}

see #776

Describe the solution you'd like
see #776

Describe alternatives you've considered
see #776

Additional context
NA

This issue is created for future enhancement to loadbalancer client customization.

[OlgaMaciaszek]

Hello, @liubao68 - we actually provide tracing solutions. Have you checked that out? In any case, I think it'd be ok to allow for custom FeignLoadBalancerClient solutions. This is not going to be a team priority, but feel free to submit a PR similar to the one you've created before, but against the main branch instead. Please make sure to add tests.