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

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 https://github.com/spring-cloud/spring-cloud-openfeign/pull/776

Describe the solution you'd like
see https://github.com/spring-cloud/spring-cloud-openfeign/pull/776

Describe alternatives you've considered
see https://github.com/spring-cloud/spring-cloud-openfeign/pull/776

Additional context
NA

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

Start in FeignClientFactoryBean and inspect FeignBlockingLoadBalancerClient and RetryableFeignBlockingLoadBalancerClient, especially their delegate handling. Review the approach described in spring-cloud-openfeign pull request 776. Done means custom Client implementations can add extensions while mixed load-balanced and URL-based clients still select the correct delegate.